We once had an old dodge and the piston rings were rusted to the cylinder walls. We put diesel fuel in each cylinder and let it soak for a few days, finally it broke loose so it could be disassembled for rebuild.
I'm not saying to do that, just sharing what we did.
How long did you let the Marvel Mystery Oil soak? (I would give it a week) I freed up a gator motor that was upside down in a creek for about six months. If it was mine, I would fill the crankcase as full as possible and remove the spark plugs as well and fill those too. If you get it to budge at all, try to work it back and forth.