I have an 1800C gas tractor and the engine is stuck. Here's what happened. I bought this tractor in the spring and did some disc work with it. It handled a 14 foot disc pretty well. Well, recently I was driving it to a buddy's house to borrow his 5 shank 3 point chisel plow when I ran into some problems. It's a good 5 mile drive and I was almost there when all of a sudden I began to get some smoke from the breather tube near the rear of the cylinder head. I was just cruising down the road in high gear so there was no huge load on the tractor. I didn't drive it very far and decided to shut it down to double check the fluids before I continued. Keep in mind the gauges don't work. A week or so prior to this happening I was preparing to add an oil pressure gauge to the tractor and had tested a port in the block and the oil came out like crazy when I fired up the tractor so I'm assuming it had at least decent oil pressure when I took off on the drive. Anyways, when I throttled it down it seemed to want to die anyways. Oil level was good. If I lost any coolant at all it wasn't much cuz the coolant level was up. When I tried to restart the tractor it was locked solid. I towed it to a nearby friend's house where it sat til I could tow it home this past sunday. When we were towing it home I tried to break it free by popping the clutch with the tractor in road gear. It didn't budge and brought my dad's truck to a halt fast. i can't turn the engine with a breaker bar on the front pulley bolt either. I have it tore all the way down to the cylinder head but it's too damn heavy to lift off there by myself. I hope to have it off in the next couple days. Should I try soaking the bores to work the pistons free? Oh, and i drained the engine oil, no debris. 
What the heck could have happened here?