Yeah but how do you wanna solve the licence problem? If someone breaks the GPL i'm sure alientrap with the help of FSF or similar would try to get those to apply to GPL by some means necessary..
Porting is hard if you can't use the code (licence) and even more so the game content like music, maps, models, textures.. As long as you can't solve this part there is no use in even thinking about the other part..