I don't know anything about your tractor engine, but I'll tell you about a mistake I made when I was a teenager trying to overhaul a six cylinder Chevrolet engine.
I replaced the rings on the pistons and the rod and main bearings on the crankshaft. When I reinstalled the pistons, I put them back into the same cylinders that they came out of, but I put some of them in backwards. There was a notch on the piston that was supposed to line up with the front of the engine - I had two of them backwards.
After I torqued the rod caps, I couldn't turn the engine over. After MUCH consternation, I realized what I had done wrong, turned those pistons around, and put the engine back together.
Just a thought.
Tom in TN