If the piston's clean of scoring, it should be OK, if the ring groove's not been worn to oversize. Just use fine steel wool to scrub the carbon off of the top of the piston. We use a grade known as "0000" (also called "four-ought" here). Hot water with regular dish detergent is all you need. Rinse with water as hot as you can stand with your bare hands, shake off, and let air dry. Oil it to reassemble into the engine.