Joe is close to it. What I wound do is put a torque wrench on the head bolts
and see just what they are. Not knowing anything more than it's an 80 cc motor, I can only guess. Honda 80 uses 22.5 psi. NOW one of two things will happen if you don't torque it right. Too loose and you will burn the head gasket. Too TIGHT and you will bust a head bolt stud. The spark plug has a gasket called a compression gasket, and thats juct what it does. Compress.
"Feel" if the plug is tight. If it were me AFTER finding out the propper torque I would remove the head then the gasket and spray it with Copper Coat. Any
auto parts store has it. JUST the gasket, nothing else! Put it back together.
See if that works