In my experience with stuck industrial motors no two were stuck the same. Pull the heads and inspect the cylinders to see how bad the rust is. Coat the cylinders with oil and let them set. I used a 5lb hammer and a block of wood to the top of the pistons to break them loose. I would take a cylinder hone to the rust areas to clean them up first. Most of the time the rings were rusted to the cylinder and once they broke loose the motor would turn over.
On extremely stuck motors I had to break the pistons apart to get it freed.
Anyway you go you are going to need to pull the heads and pan as you will find that rust will be on the crank journals and the rods. I find it highly unlikely you will be able to just free it and make it run. Good luck...
