Somehow I found this site:

http://members.aol.com/westernstar66/indianlisters.html

that lists a 20 hp diesel for $3250, a real bargain for a new engine
compared to most diesels for boats. These are low rpm diesels that
can run on vegetable oil and are very efficient.

So, what would you do for a transmission? Maybe you could use a belt
with an idler pulley that was lever controlled. Tension on the idler
pulley would sorta regulate the speed. Maybe the engine has a speed
control, I dunno. Reverse would be a real problem. These look heavy
but just cool as hell. I love simple stuff, great engineering.