[piercebot]

Are there any copyright concerns around doing this kind of thing? Surely SEGA will come after him for providing this as a free download?

[wk_end]

It's provided as a patch - you still need to procure the original ROM yourself (legally or otherwise, but that's on you). Someone more knowledgable in copyright law can discuss the specifics, but AFAIK this is totally legal. It hasn't been any kind of problem for the ROM hacking scene so far, anyway.

[reificator]

It's distributed as a patch file, and also he founded and currently serves as director of the company that originally developed the game. I think he'll be fine.

[spicybright]

And just to add, Sega has been historically really relaxed about sonic fan games.

There's even events just for sonic rom hacking that happen yearly like Sage: https://twitter.com/sagexpo