One that I’m aware of is Re-Volt. The source code for both the base game and some addons can be found here.

There was another one that I remember using years ago but I can’t find it anymore. It was for another racing game called 4x4 Evo. There is the 4x4 Evolution Revival project but it’s not the one I used and there doesn’t seem to be any source code available.

I got How to Get Bulbasaur in Pokémon Yellow, so I guess I should probably avoid that game for a while.