There are cultivators with a worm gear drive using low hp motors, ratios depend on rpm and engine size. I've seen reduction ratios quoted as between 20:1 and 42:1.
Another solution is to get hold of something I only ever see advertised for sale in Australia. Hand held petrol angle grinders. They use a standard clutch assembly and a 90 degree bevel box. "All" that has to be done then is to reduce the speed of any shaft you fit to it, and find a way to turn the back wheel.