You could try putting the warped chassis in hot, not boiling, water and then try laying it on a flat surface with a weight on it until it cools to the proper shape. OR You could try to make your own flat, promo style chassis out of thick sheet plastic cut to the same size and shape of the promo chassis. You could try to make your own axles to hold the wheels/tires out of either brass, aluminum or plastic rod or tubing, which you would use either CA glue or epoxy to attach to the chassis at the correct wheelbase spots.