Are you sure there's pinch?....He's talking about a Baja 5B engine...that's a gasoline engine with a piston ring...shouldn't be any pinch.

Piston should never get stuck unless it is so flooded the combustion chamber is full of fuel and the piston won't move any further. In that case, take the sparkplug out and crank it over a few times.

If it's really stuck.....remove the exhaust and look at side of piston through exhaust port. Do you see any damage?...Like heavy scoring? If so, it has to come apart for repairs.

If the piston really is stuck due to heat, then be careful about heating the cylinder head.....there's gasoline around, so don't use an open flame. Try a hair drier. I can only imagine a piston being stuck this way if the engine was running way too lean and got very hot and after shut-down, teh ring/piston somehow stuck to teh cylinder wall.....but the whole thing sounds unlikely to me.