have you considerate to use another game engine instead of full conversion? There is a FOSS game engine called Xreal which is based on idTech 3 (quake 3 engine), you can use the src code and yet compile for many platforms (linux, Mac OSx, OpenSolaris and others). I hope that you guys could make a sand-alone game for at least win, linux and mac os x. Oh yeah just in case all devtools are provided also its respective source code.