I haven't look at this game in particular, but usually you need to buy the original game to get the assests (art, models, audio, etc). Even official open-sourc

I haven't look at this game in particular, but usually you need to buy the original game to get the assests (art, models, audio, etc). Even official open-source releases like the Quake series still require you to buy the original game in order to run them.