Great work at all, here is the comment. You can make the battle field longer with more different shapes and set a destroy key in both of the marble circle like [Enter] in the "Controler" section for the singal to start the fight!
It seems more problem than your pervious work, the cars may stuck on the track because of the friction, you can reduce it or set it to zero. Also, the bouncy bar's restitution is too large as the car may fly out the track.
