It most def has potential to build as a racer. The auto, or otherwise known as a hydrostatic trans in that machine wouldn't be something you'd use anyway. You need a kart axle or some sort of geared transaxle (not hydrostatic). If you do an axle with a sprocket on it, you'll need a transmission. Lots of people use the Peerless 700 transmission. Of course, theres lots of work to be done to retrofit a live axle and a Peerless trans into a machine that was originally equipped with a hydro axle. Check out some of the builds in the project section here and you can get some ideas. I built a racer out of a brand new Cub Cadet (in the build project section) and it was a hydro axle and I used a kart axle and Peerless 700. Check it out, maybe you can get some ideas. Please don't forget about safety! These machines were designed for 5mph....not 50.
~Joe