If you are lucky... (wish everyone was)
Remove glow plugs.
Pour in a small amount of 50/50 mixed ATF and Acetone. Turn engine so the pistons are down if possible.
Lightly screw in the glow plugs to prevent evaporation.
Let sit for a day or two or three.
Remove glow plugs.
Try to pry the crank over a bit. Work it back and forth.
Over the years I've broken free some massively stuck engines successfully. Usually a combination of things work to free them including the Acetone/ATF trick. If the head is off, light tapping on the piston can help too--the shocks help vibrate things loose.
But once it moves a tiny bit, just go back and forth and things should free up.
(instead of ATF/Acetone, you can use Kroil, which is excellent too... Both WD-40 and PB don't work worth a hoot, IMHO, and should not be even tried.)