I had a similar problem with a car years ago and had collected a variety of things to try to get it unstuck. Read to the bottom to see how it turned out.

1) popping the clutch on my steep driveway.
2) Putting a board under the hand crank and bouncing on the bumper
3) Marvel Mistory Oil in the Cylinders and let it soak
4) Warm the block with a propane torch, then quickly cooling it (then go to option 2)
5) WD40 (or other super penetrant) in the cylinders

In the end... nothing had worked for me as number 3 and 6 were stuck badly on my inline 6. We ended up taking the engine out, removing the top and bottom ends and driving the pistons out with a hammer. The rings were so badly rusted to the walls that it would have been impossible any other way.
Jerome