What you need to rewrite depends a game. I would say that with Descent there will be quite fwe SDL stuff left, you will need to rewrite 3D routnes anyway, with Zelda it could be much easier and more meaningful.
But please note, that 68k is very weak cpu nowadays, so every frame counts. I really wish that you succeed, but I have some doubts how it will perform.
Nothing is more annoying that app/game that works... almost. They are just proof of consepts, not a usefull games/apps. Another point is that you purpose is port software for real amigas and is not meaningful to have game that requires 060/64mb when it should work just fine with 030/16mb
