I repowered an old Cletrac OC3 with a 74 Mustang 4 cyl engine. It was quite simple. Mounted the engine in the frame, machined an adapter to match the Ford trans input shaft to the Cletrac transmission, started it up and drove it away. It was the fastest OC3 in the East. I sold it probably 20 years ago and the last I knew it was still going strong 2-3 yrs ago skidding firewood, plowing and light dozing.