I have a horse tiller (serial 309638) that is a 2 speed with a 2 belt setup. the large pulley has 3 "v" grooves. 2 the same size for the belts, and one larger diameter one. The small engine pulley only has the 2 v grooves. How does this particular tiller go into reverse? There is no flat spot on the large pulley for a reverse disc, and no reverse disc either. I bought this tiller apart so trying to get it back together. Can I convert it to a 4 speed with proper pulleys?