for slow-speed like 650 rpm you might be stuck with the heavy iron antiques like the 3 1/2 - 1 or larger. Maybe the Lister Junior.
The newer air-cooled stuff is available down in that horsepower range and some tidy little packages. LD, SL, LR are examples. These are typically not rated to run under 1,000 rpm though. I believe that their practical loaded ratings start at about 1200 rpm. horsepower ranges are about 3hp to 4 1/2hp and are designed to be run up to much higher speeds if more power is required.
dieselgman