What's your budget? You can buy on ebay an electric motor with a built-in geared reduction to just about any outout rpm. I just got a used one for a hay elevator for about $75. Free shipping.
If low budget, go to a junk yard and look for gas motors from a lawnmower which was self propelled. The output shaft on these have a sprocket to chain drive your unit. A gas rototiller unit will work, too.
If you can get a 12v starter motor hooked up to it, you could run it off the trucks battery for a few minutes with a voltage controller off a golf cart.