The source code for a decompilation of a game can only be available legally because someone who isn't the original dev created the decomp and decided to share it publicly. All assets in the game were still created by the original developers, and can't be freely shared. This means the open source decompilation will need a way to load assets from the original game to remain legal to redistribute. This includes things like textures, 3d models, and music.

Chances are, once this project is completed, you'll need an original copy of the game to play, and it'll use the cars, maps, and music from the original game.