Have a picture of the frame you're mounting it on? The GX120 should have the standard PTO mounting holes in the side cover. You can use a piece of 1" flat stock steel and a skateboard wheel. A hole drilled in one end to mount to the cover and one in the other to mount the wheel. Attach a spring and put the other end of the spring attached at the cylinder head. That will keep upward tension on the lower side of the chain.