Since you already have the heads off, you might want to try pouring a mixture of 50/50 acetone and automatic transmission
fluid into the stuck cylinders and let it sit for a few days. If it drains thru into the crankcase then pump
out and re-pour into the cylinders. Then take a 2by4 or any piece of wood
that will fit down into the cylinder and give it a bunch of whacks with a sledge hammer or lead hammer. Freeing up a rusted diesel
is not a gentle process, and takes some big effort. If you can bolt some kind of a bar to the flywheel and try moving it back and forth until the piston begins to move, That is another option. The third method (use all three) is to take a propane
torch (after the acetone mixture is removed and a fire extinguisher is handy) and heat the top of the piston as hot as you think you can and then blow cold air on it. The expansion and contraction helps to break the rust loose. If you get it freed up, you can pull the pistons and see if the barrel needs to be bored or if the pistons are damaged from the rough treatment, or if the rings are broken, or if you are into risk taking you can put the heads back on and see if it runs without too much blue smoke. I have freed up a number of old diesels using these methods and sometimes the results are good, and sometimes not good, but a seized diesel
is no good for anything but ballast. Good Luck with it._____Grant.