The original engine cannot handle more than one opponent at a time. Even if it were modded, it'd be really hard to implement more opponents. In memory, there's the space to load the information for "THE" current opponent and there's only one designated started position for him/her. A new engine should be created from scratch or an enormous amount of work should be put on modding the current one. With a new engine, several opponents at a time are not hard to implement. Much harder it is to simply create a fully functional collision mechanism, which is what got me stuck with the engine I've been making.
As Daniël said, cars are separate and it's more straightforward to edit them. They collision information is also very simple. In contrast, track elements have some information hard-coded in the game and the collision info that's not hard-coded is not very well understood yet and appears difficult to edit. There have been a number of visual modifications to the current 3D shapes. That can be done with Stressed.