The processes of breaking in an engine is no more than seating the piston into the sleeve. Running it hard without doing a break in wearing heavily on the piston and sleeve. When this happens the piston and sleeve don't fit how they were designed to fit when the engine gets up to operating temperatures. Parts that don't fit as they should cause airleaks, which means less compression and there for much harder to start, keep running, harder to tune, and the pressence of "flame outs" (push on the throttle and the engine dies).

A rebuild and make an engine like new, if it's done right. Doing a rebuild right required a CLEAN work area and some rubbing alcohol. the rubbing alcohol is just to clean the parts and remove small bits of dust, dirt, or anything else. After the engine has the new piston and sleeve in it, a fresh glow plug isn't a bad idea either because you'll need to BREAK IN the engine this time to avoid having to rebuild it again anytime soon.