AFAIK, the closest thing to a mod that is allowable is an insim system on a server. You are not allowed to edit the cars, tracks, etc. I do believe that texture mods are allowed for tracks. And custom skins are obviously allowed. But modding cars is not.
As Learjet said you are not to allowed to change the shape of the cars body, you can however change the engine sound, car interior textures, track textures etc, you will find plenty of them in here