Mike and JR:
My dad said when the tractor quit running, my grandpa left it outside for a year before he pulled it in the shed. The engine was still free at that point. There it sat in a steel, uninsulated pole building out of the Michigan elements.
A little more info on what I've done and my observations. I drained the oil to flush the crankcase, there was a little water, but nowhere near what I would have expected. I removed the valve cover and everything looked clean, valves all moved, though one stuck a little at first. Pushrods also seemed good and were not sticking.
I removed the cover to the crankcase. Piston on clutch side is near top dead center, a few small spots of surface rust on the cylinder wall. My thought is this one is stuck. Flywheel side piston is near bottom of cylinder, I can see where my penetrating fluid has leaked through and ran into the crankcase, leads me to believe a ring job is certain.
There is a little wiggle in the flywheel, but not much. Just enough to see the bearings on the crank are not stuck. Overall, I'm impressed and surprised with the condition of things considering how long it has sat.