Modding a Racing game and connect to external DLL

The project is to make a simple mod of a racing game that changes according to our DLL. The game communicates with our external DLL to decide on some effects during the game. Any Racing game with good graphics will do, though it should be based on a common game engine. game effects that are changing while playing are "regular" effects, such as speed, light, sound and others. full game-change description, API and DLL will be supplied.