Who could ever forget seeing for the first time the film Harry Potter and the Sorcerer's Stone and watching Rubeus Hagrid bring to Privet Drive baby Harry Potter on a flying motorbike? Not only did this particular motorbike make its way back into Harry Potter and the Deathly Hallows (closing the circle, as it were) but it also has made its way into Harry Potter FanFiction. Of the Harry Potter FanFiction that I have read, whenever the 'flying motorbike' is part of the story, it most always has been depicted with the Marauders, post-Hogwarts. Of course, these FanFiction stories would combine the 'flying motorbike' with the Marauders, being that the motorbike belonged to one of the Marauders, Sirius Black, as readers of the Harry Potter series were first informed in Harry Potter and the Sorcerer's Stone.
Dumbledore - "And where did you get that motorcycle?"
Hagrid - "Borrowed it. Young Sirius Black lent it t me.
As readers, we know how Hagrid acquired the 'flying motorbike,' but how did Sirius Black acquire it?
A very simple explanation is that Sirius Black merely found a motorbike that he liked, and with a little bit of magic, he was able to make it fly. And this theory is plausible. Within Chapter 10, Kreacher's Tale, of Harry Potter and the Deathly Hallows, Harry, along with the reader, learns of the fondness that Sirius Black possessed for Gryffindor House, 'bikini-clad Muggle girls,' and Muggle motorcycles.
"The teenage Sirius had plastered the walls with so many posters and pictures that little of the walls' silver-gray silk was visible. [...] There were several large Gryffindor banners, faded scarlet and gold [...] There were many pictures of Muggle motorcycles, [....]" HPDH (Chapter 10-Pg. 178)
Along with creating the Marauder's Map and becoming an Animagus, even Dumbledore admits that it is an 'extraordinary achievement' that the Marauders were able to become Animagi and perform difficult magic at such a young age. In other words, Sirius Black, and the rest of the Marauders, would have had the magical capability to enchant a motorbike so that it would fly.
However, there exists another plausible theory to the origin of Sirius Black's motorbike....
He acquired it from Arthur Weasley!
Think about it.... The 'flying motorbike' is not the only flying vehicle that both Harry potter and readers of the Harry Potter series have seen. During Harry's second year at Hogwarts, Harry Potter and his best mate Ron Weasley miss the Hogwarts Express and decide to travel to Hogwarts in the Flying Ford Anglia.
And who owned that car? And enchanted it so that it could fly?
So, based upon this train-of-thought, there does exist some plausibility that Arthur Weasley could have enchanted the motorbike, being that readers of the Harry Potter series are witnesses to the successful attempt of the enchantments of Arthur Weasley upon his Ford Anglia. Yet, if this plausibility is to bear any merit, how did the 'flying motorbike' end up in the possession of Sirius Black?
We know as readers that Sirius Black was in possession of his 'flying motorbike' sometime before the downfall of the Dark Lord, knowing that Hagrid used the motorbike to transport baby Harry Potter to Privet Drive. In other words, if Arthur Weasley did give Sirius Black the 'flying motorbike,' then there has need of being a connection between the two prior to the downfall of the Dark Lord.
During the Second Wizarding War, other than Harry Potter, the Weasleys and Sirius Black were connected by means of the Order of the Phoenix. However, during the First Wizarding War, though Sirius Black was in the Order of the Phoenix, neither Arthur Weasley nor Molly Weasley were in the Order.
However, the brother-in-laws of Arthur Weasley, brothers of Molly Weasley, Fabian and Gideon Prewett, were in the Order of the Phoenix during the First Wizarding War.
Prior to Number 12 Grimmauld Place being used as headquarters for the Order of the Phoenix, it is not entirely known if there was a headquarters for the Order during the First Wizarding War. It is unlikely that the home of a non-Order member would be used as headquarters. But, it is likely that the home of a non-Order member could be used as an occasional meeting place for the Order. This is able to be a possibility by knowing that the Order has used non-Order member homes for purposes of the Order. In Harry Potter and the Deathly Hallows, Chapter 4. when the Seven Potters prepare themselves to depart from Privet Drive, Alastor 'Mad-Eye' Moody informs Harry of where the first rendez-vous points are prior to meeting up at the Burrow.
"[W]e've given a dozen different houses every protection we can throw at them. They all look like they could be the place we're going to hide you, they've all got some connection with the Order: my house, Kingsley's place, Molly's Aunt Muriel's - [...] You'll be going to Tonks's parents." -Mad-Eye Moody (Pg. 48)
Places like Molly's Aunt Muriel's and Tonks's parents are not direct homes of anyone in the Order, but they do have some connection with the Order. During the First Wizarding War, though Arthur and Molly Weasley were not in the Order, they would have a connection to it, being that Fabian and Gideon Prewett were part of the Order. Therefore, the home of Arthur and Molly Weasley could have been used as a meeting place for the Order of the Phoenix, allowing Sirius Black to be there and acquire the 'flying motorbike.'
Another, and extremely simple, explanation of how Sirius could have been at the home of Arthur and Molly Weasley is that because Fabian and Gideon were the brothers of Molly, they merely could have been visiting their sister. In this scenario, the two brothers either brought along Sirius with them, or Sirius had need of informing the Prewett brothers of something, thus causing his need to be at the Weasley residence.
Either way, there still exists the connection between Arthur Weasley and Sirius Black, by means of the Prewett brothers being members of the Order of the Phoenix.
If Arthur Weasley did give Sirius Black the 'flying motorbike,' with the only connection between the two being the Prewett brothers, why would Arthur give the motorbike to Sirius? The two of them were not close, and based upon what Dumbledore asks Hagrid regarding the motorbike in Sorcerer's Stone ("And where did you get that motorcycle?"), it was not intended for the purposes of the Order. So, again, why would Arthur Weasley give Sirius Black the 'flying motorbike?'
The most reasonable answer is that Arthur Weasley had no choice. A lack of choice not because Sirius Black forced Arthur into giving him the bike, but rather because what Arthur was doing was illegal.
Being that he works for the Department of Misuse of Muggle Artifacts at the Ministry of Magic, Arthur Weasley knows very well that what he is doing is not considered legal, being that he is enchanting a Muggle artifact to possess magical qualities.
Just imagine that Arthur Weasley one day or night is in his shed and he is working on a motorbike; to be more specific, he is enchanting the motorbike so that it will fly. Then, either because he is with the Order, with the Prewetts, or stopped by because the Order or Prewetts were there at the Weasley residence, into the shed walks Sirius Black and discovers what Arthur is doing. Knowing the Prewetts, Sirius is not going to turn in Arthur to Ministry authorities. However, being that Sirius has a fondness for Muggle motorbikes, Sirius could convince Arthur to let him have the 'flying motorbike.' Sirius (in my opinion) would never blackmail or bribe Arthur; but he could very well have said to Arthur something along the lines of him being able to own a Muggle motorbike with magical abilities whilst Arthur would still be able to observe the success of his enchantments, along with not having to worry out being caught owning it.
With regards to owning a 'flying motorbike,' or any other enchanted Muggle artifact, there does exist a 'loophole in the law.
Arthur Weasley - "There's a loophole in the law, you'll find.... As long as he wasn't intending to fly the car, the fact that the car could fly wouldn't -"
Molly Weasley - "Arthur Weasley, you made sure there was a loophole when you wrote that law!"
(Harry Potter and the Chamber of Secrets-Chapter 3-Pg. 39)
Yet, perhaps this law was not written by Arthur Weasley prior to the downfall of the Dark Lord. And perhaps that it is this event of Arthur giving Sirius Black the 'flying motorbike' that inspires Arthur to write the law, which then would allow him to have a loophole within it and be able to own an enchanted Muggle artifact again in the future, such as the flying Ford Anglia.
Hopefully this Origin Theory of Sirius Black's Motorbike offers a new perspective to another artifact within the Harry Potter series, as well as leaving all of us with the curiosity to explore the origins of other artifacts.