Paint the calipers metallic gray, glue to the rotor and install in the finished wheel. The wheels on my shelf models do not turn so it's easy to rotate the calipers to the correct position when installing the wheels but those used on slot cars will have to rotate with the wheels. Drum brakes, used on earlier racers and many Ferraris as they were the last to convert to discs, can be modeled by installing just the washer behind the wheels. Use a slightly larger diameter washer than the ones used for the disc brake rotors as the drums were as large as would fit in the wheels - as the outside diameter increases, so will the inside so grinding may not be required in this case. These were usually painted the same color as the chassis or the body.
Modern racers use vented discs of larger diameter than the classic style.
FLY's OZ wheels as used on the Marcos look fantastic with the drilled rotors because they are the most visible. I dispensed with calipers because there isn't room to install them. Remove all four tires from the wheel set, taking care not to wear the markings off if possible. This gives you an even guide to use in cutting the outer rim and spokes from the main part of the wheel. There should still be plenty of hub left for strength after cutting.