You can replace or modify body panels, but the overall dimensions and wheelbase need to stay full-scale or changed only minimally by other allowed modifications. You can replace or modify subframes in order to do other allowed modifications. You can do fancy stuff like add fasteners to attach bodywork, and you can remove fenders and running boards from pre-war cars. No matter what you do, no part of the car other than the wheels and tires can be less than 3 inches from the ground.