If all else fails take a good solid wood 4X4 and trim the corners as necessary to fit comfortably in the cylinder. cut the length to maybe 6 or 8 inches above the block. Or find a good solid wood post of the appropriate diameter. Hit the wood on top of the piston with a SLEDGE hammer until the piston goes down maybe a half inch, then clean inside the cylinder above the piston as necessary.
After the piston moves down it should be loose. take a metal rod and hit on the bottom of the piston in different places, sparing the connecting rod to remove the piston. As mentioned you will likely need new pistons anyway.
I said if all else fails use this method. You can ask me how I know this.
