OK here’s what the problem was. I pulled the head and found water on top of the cylinders, and it was water not antifreeze. After toweling out the water and shooting a little WD 40 on the cylinder walls, the engine spun free. There was no sign of rust on the cylinder walls or on the cylinders, There was a little flash rust around the exhaust valves, so I decided to lap the valves while I had the head off.
I really have no idea how it managed to get water in. We have had some very heavy rains here lately. My manifold is in rather poor shape, there are little to no threads left where the exhaust pipe threads in, so I don’t know if water managed to seep in that way. I have a muffler on with a flap, so I really can’t figure out how the water got in there. I did buy a new manifold and exhaust pipe . This had to have happened over a period of about six weeks because that is as long as the tractor had sat without being ran. The water was really not in long enough to do any kind of rust damage but it corroded the cylinders enough to bind the pistons.