It's not possible, simple as that.
What's not possible is to make the game the same. Didn't I say that in my previous post?
The number of enemies etc. would have to be stripped down extremely.
How much? You can have a few dozen software sprites on screen at any one time. The problem is more with that flame thrower.
the music would probably have to be remade using 4 Paula channels, because for ogg-music there won't be CPU time
Easy. Down sample to 28khz, and store as native 12 bit stereo. Now you can also mix in three sound effects per stereo side (hence the 12 bit and not 14). The audio files will end up big, but who cares? It's not as if it would have to run off a floppy disk.
Basically, the problem I have with this is that you're dismissing it completely without thinking it through completely (that's what it looks like). You're just saying it's completely impossible without really knowing how close you could get (again, that's what it looks like).
That said, I'm not saying anyone should try it, but it would be nice to find out how close you'd be able to get (always much closer than nay-sayers would have you believe, of course).