[Lynn Eades]

If you sand out the rust, and then pour some diesel in each cylinder and set fire to the diesel, the heat from the slow burning fuel will warm things up, and probably free it up. You may even get enough heat from the little bit that sits around the outside of the piston with the hole in it. If that doesnt work, drill the hole and tap a bolt into it, to stop the diesel running through.
The diesel may have to burn for maybe an houror more. Try it!