The repo is basically Quake II's software-renderer and game-logic module merged into the engine binary. Yamagi Quake II ships with an additional OpenGL 1.3 and 3.X renderer which I'm sure could be ported over if SW was undesired.

While it's something cool to work with Quake II, its net-protocol has its downsides and I'd encourage anyone replace it with QuakeWorlds' networking as you'll run at a locked 10 Hz with the clients interpolating between the frames otherwise. This was done for easier handling of animations in single-player entities, but was regretted later (according to JohnC's own .plan files).

Quake 1 is nicer to work with in that regard. Also get used to wobbly model animations with the MD2 model-format as the precision of the vertex-animated models is very poor :)

It's also interesting to note that Carmack considered using a 16-bit MMX-powered SW renderer for QuakeWorld/Quake II (making it real cutting edge in 1997), but considering the global 8-bit palette was already in place, it would have resulted in a loss of precision of already paletted textures so it was decided against and they did it the old way.