Honestly there is no challenge - technically, not game play wise - to make Sonic for a stock (!) a1200 as the latter is the faster and better hw machine, so it's nothing much of an achievment.
If you really just seek challenge make it work on a stock (!) a500 now that is challenge, as an a500 has weaknesses and strengths vs mgd (slow bitmaps vs fast character/tile maps for one, but 4096 colours palette vs 512 colours and so on).
But It seemed this project is not a "geek demo compo", but making of something that anyone would love to play, a new Amiga (preferably Amiga only!) game...
Well... never mind.