I mean hell all you’d really even need to make it moddable is to have the source code around and keep it documented. The reason the originals are so difficult to mod is you’re basically hacking up compiled code via trial-and-error.
I just remembered a similar project, by the way: a Sonic fan
made his own engine and ended up being hired by Sega to do total remakes of the original games for iOS. I’ve played them, and they’re 1:1 true to the originals but in an entirely new engine instead of Sega’s usual “stick it in a Genesis emulator” shtick. It let them build in unlock able new characters, edit the levels to add secret areas for Tails and Knuckles to reach, run the game in widescreen, and just generally be a better version of the games.