If you can find some, a product called Croil is good. Croil is an abbreviation for Creeping Oil.

I would be very careful not to pound on the piston head as this will only make the piston tighter as the dome flattens out from the percussion's. If the piston is stuck soo tight that it cannot be tapped out or pressed out, I would recommend drilling and carefully cutting the piston to relieve the stress and then removing it. The bottom skirt of the cylinder is very fragile and can crack and break. It is less expensive to replace a piston and bore a cylinder than to replace both piston and cylinder.

Good Luck