We are not in year 2004 anymore. Check out http://sauerbraten.org/ . Its cube 2 ;D
anyways: editing the graphics will lead to far better results on current engines wont it?
btw for a content mod check out data/default_map_settings.cfg and data/default_mapmodels.cfg, useable model-fileformats are md2, md3, md5 and obj
textures are saved as jpg or dds.
to modify the code either use codeblocks, gcc or ms visual studio (on windows either codeblocks or ms visual studio, but the codeblocks project file is working whereas the visual studio project needs some little tweaks and updates to the correct paths to work, so better use codeblocks as newbie). just dive into the code. :)

gl hf