After about a year of hard running they will have developed some soft carbon build up but it's pretty much a wipe away thing if desired. Piston wear has usually only been notable when foreign object injestion has occurred. Not using an air filter means anythng that gets sucked into a carb makes to the inside of an engine, so small scratches happen from time to time. As in any other engine there will be light "scuffs" on a piston where they arc a tiny amount inside the cylinder during directional changes.