It may be stuck to where you have to tear it down, but taking the spark plugs out and squirting Marvel Mystery Oil in each hole and let it set for several weeks might help. To turn it I would take the tin cover off the flywheel housing, use a pry bar on the flywheel ring gear to try to turn the starter ring gear. Try it both ways. Sometimes you can get them to move a little one way and then a little the other way until you can get a complete revolution. If this doesn't work, taking the head off and using a wood 2x4 as a punch, hit it with a hammer. If this doesn't free it up, you probably will need a total overhaul.