Head gasket, possibility. Also, you replaced the piston...Might be a sealing issue with the base of the cylinder to the cases being the coolant flows thru the cylinder.
That said, coolant may be leaking thru the base gasket into the lower end..
Pull the plug and see if there is coolant on the plug.
If I am correct, even with aftermarket head gaskets, the smaller coolant holes align with the exhaust port side, even tho there may not be an "E" stamped on the gasket.
I don't have a head gasket handy to snap a pic. and show.