Stuck engines happen all the time, in tractors. The main reason is being left outside w/o a covering on the exhaust and the bores becoming rusted.
If that's the case, the way I did it, was to make a mixture of 1 part solvent, 1 part ATF and 1 part Xylene. Pull the plugs and put as much as it will hold, in each cylinder, wait 2 days and then use a large screw driver or pry bar, on the flywheel teeth. If it moves, back it up and go, back and forth. If not, wait 2 more days and try again.
Tractors are inline engines and I broke loose a 6 cylinder that way, that had set, 10 years with a vertical exhaust and no covering on it. Because it was a inline and the solution would cover the piston, I only used 3 oz, per cyl.
Xylene is one of the most penetrating solvents (better than acetone) and what it does is, pull the thinned ATF, down into the rust, to loosen and lubricate. Toluene, is another solvent, that penetrates better than acetone but, not quite as well as Xylene.
It took me 2 hours, after it first moved, to get approximately, 2 full rotations. After that, I put the starter on and was able to spin it normally. The bad part is, that it had a max of 7 lb compression.
If it's rusted and you can't get it to move after the soaking, you have to pull the heads and pan, loosen the mains and oil them, try moving the lifters first, get them out of the way, then pull the easiest piston to get at, that will go between the crank and block, keep trying to turn the crank. I did a 302 that way, it took me 2 weeks of frustration, working about 2 hours a day on it but, I got it apart.
Frank