new piston, hone bore, or new wrist clip
Piston slap means something has worn and the connecting rods are now hitting the cylinder. like
the first is the wrispin which connects the piston to the rod. The second reason could be the rod barring itself.
personally i think it'll be just valves or loose bolt somewhere near the engine, unless its smoking.
if its not take it to a mechanic for a check.