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.
David