Simplicity rotor tiller OLD

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I have a Simplicity rotor till about 1940's vintage. It has a 6hp B&S motor. I remove the tiller when not needed and put a 30 inch mower in front and it works great. Could use a bit more power. I am a newbie to this site. My question is about the drive system. It has 3 forward gears and one reverse. The gearbox drives a couple of massive gears in grease to get to the axle. The axle is solid. At the end of each axel is a strange device that drives the wheels. One of these units is missing and I was wondering if the parts are available and where. Thank you BC Mike C