Would have to be bad head gasket, it's leaking and getting down the cylinder and when you kick it over it's blowing the antifreeze right outta the exhaust port is my only explanation.

lap the cylinder and the head before install a new gasket  ! i suggest only use oem gasket. E mark will face the Exhaust.....

Get a surface plate from amazon and 600 grid sandpaper Remove the stud on the cylinder and sand on the surface plate the head and cylinder until the surface are totally flat and free of dent. When putting the gasket it supposed to have an E mark on the gasket marking the exhaust side. Also you can use gasket spray with the OEM gasket. Torque the nut to the correct specification and recheck torque after each first 3 hour of operation. 

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.