AFAIK the FP's all used a cast iron piston and steel liner so should have been run with castor in the fuel. If so then the castor is gummed up and I've found acetone to be the fastest thing to loosen gummy castor but once freed up you need to flush it with fuel before the acetone evaporates to keep it free. The one thing that nothing (including acetone) can get at is the wrist pin area. If that's gummed up, and it will be, you need to remove the back plate, invert the engine and fill the inside of the piston with acetone or fuel then try sliding the rod along the wrist pin until it comes free. I use the short end of a small allen key to pull the rod back then push with a finger. If this isn't freed up then the wrist pin will rotate in the piston and wear out the piston holes.