Keep running it unless its starts smoking an using oil. Piston slap is where piston is worn from dirty air filter or dirt getting into engine. Skirt of piston wears out an starts slapping cyclinder walls every time it fires . Only problem is if it wears enough it will break off an damage rest of engine. Had that happen in two strokes. Yours probably never do because it don t turn too many rpms. Don t take 3 hrs to down either. Maybe hr an most time have install new gaskets to put back together . Another thing if it wears out so bad an piston breaks it will damage the plating on cyclinder wall. Most all newer atvs an bike have a nikisil planting to keep it from wearing out . Piston always wears out before cyclinder. I would just ride an not worry bout all that . Piston slap sounds just like a valve rattling . Hard tell difference. Like someone else said could be slack in timing chain also. I have a cr250 an the piston starts slapping bout 20 hrs an i keep filter clean.I have put 100 hrs on it before i rebuild it. Even if piston is slapping just ride it as long its not smoking an using oil.