Pull the cylinder and measure the bore size. It's possible that the bore is fine but the PO installed the wrong size piston. You're pushing your luck running anything over .005 clearance. Piston slap is the action of the piston skirt hammering back and forth in the cylinder. A piston will only tolerate this for so long before the skit breaks off, becomes wedged between the crank web and the case, then destroys everything from the piston to the cases. The quad doesn't know how much you paid for it or that it's not going to be raced. It's a machine with a serious problem that will not hesitate to self destruct and leave you stranded 12 miles into a ride if you don't fix it. Best bet is to bite the bullet and fix it now while it can still be fixed. A top end, bore, and gaskets can be had for under $200. An engine can run you over $800.