More than likely the head gasket (even though it may look good) is the usually main culprit followed by a cracked cylinder, or a cracked head. Since it sat between uses it doesn't take much of a leak over time to fill a cylinder, since you don't appear to have coolant in the oil pan you may if you already haven't drained the engine oil, any coolant will usually come out first since it's heavier than oil. A engine machine shop that specializes in diesel engines "should" be able to pressure test the head, and block, or you can have them checked it for cracks by having them magnifluxed. If you didn't have any overheating problems the head gasket would be the most likely. Most head, and block cracks are usually from overheating (or freeze if the coolant is weak). For the news you may not want to hear is finding parts maybe difficult since the tractor was built by Toyosha, they also made tractors for Duetz-Allis, Simplicity, Allis Chalmers, and under their own brand Hinomoto, but have since quit making tractors for years. The MF head gasket part number is 3281473M1, the other gaskets you can make yourself.
