To complete the final class (A class) easily, go to restart the game. The car body design that you had used for the A class races is the same, and not back to normal. All the parts, accessories, wheels, shape, and body parts are all the higher class parts. If you made a good car and want to use it in C class, you can. Do A class, then start a new game (do not close the game and open it again or you will just get the standard items). You will get the same shape car you had previously, but the performance will be different.