Marathon is a triology of ego shooters of Mac os 7/8/9 made by the team (bungie) that made Halo for XBox later (there are halo demos 4 macs!) cause m$ bought them
Marathon worked flawlessly on 68k macs (25-66 Mhz) in 640x480 speed should not be the problem.
AlephOne is a OpenGL Port of the Marathon source. Brought the game to win and linux.
The source is in c++.
As far as I can see in the code the pics are rendered in 320x240 and blew up to 640x480 cause some 68k macs had a hardware zoom. (looks fine on gp32?)
Problem: GP32 lacks some controls->some features have to be brougt to a "in game menu" etc.
If there are some people (preferrably mac os 9 c++ programmer and arm assembler coders) out there who would like to port, i could help a little. Its too big for one!