you could try the labour saving way to clean out the crankcase, repeated filling / shaking about and emptying of the case with petrol using a fresh charge each time, you'll get a feeling if the crank bearings are clean by rotating the crank by hand while holding the con-rod, It'll be an engine out job of course. Most of the damaged piston will have exited via the exhaust.
so what size jet is fitted right now? I'm thinking a 250 will be about right.
Take the reed assy aout and make sure thats clean too