I bought a t bucket last year that had been sitting 13 years, it was stuck.
Put lots of PB Blaster in the cylinders, let it sit. After a couple days I tried to turn it with a breaker bar and socket on the crank. Wouldn't "turn", but would wiggle just a tiny bit. Just the rod bearing play. After a day or two more, it would move just a bit more and I knew pustons were beginning to move. After a couple more days and try's, I could get a whole revolution. Pretty soon it would turn close to normal. By the way, I never put anyway near enough twist on it to break anything.
It started and is a great, solid runner now.
My lesson, which may or may not be of any value to others, is that getting one unstuck is not like getting a bolt to turn. Break it loose and you're done. It's a slow process. Work it loose. Be patient.
If you can move it at all your chances are pretty good. It's unlikely it's rusted up, or at least not badly.
Another trick is heat. Some take small motors and put them on a hot plate to break loose varnish, etc. I'm not sure exactly how you'd do this with a big motor like ours, but the concept seems reasonable. Maybe a salamander (kerosene) heater directed on it?
It didn't get stuck in a few minutes, there's a good chance it won't come unstuck that quick.