Before you dig too deeply into that engine, ask for advice on boatdiesel.com. Often rings stuck to the cylinder bores can be freed with penetrating oil
and barring the engine back and forth. If that fails, pull the head
, drop the pan and remove the con rod caps. Try rotating the engine then as one or two of the cranks will be free to rotate one way or the other. Then pound on the piston top with a wood drift.
I haven't done any of this myself, but I have read about it on boatdiesel.