Comment Industrial engines (Score 1) 543
Don't discount industrial engines. While automotive diesels are designed to operate at a wide range of RPMs, industrial engines are typically designed for a fixed speed purpose, such as powering a water pump to lift water from one canal to another for irrigation, or powering a generator, etc. An engine like this with a well tuned intake and exhaust can be extremely efficient and extremely quiet.
Google "Helmholtz Resonator Principle" for designing the intake and exhaust properly.
One that pops immediately to mind is a lightweight, but very torquey 3-cylinder Kubota Diesel engine, such as that used in the Urba-Centurion (128mpg DIY Diesel sports-car designed by Mechanix Illustrated back in the early '80s).
If that one isn't powerful enough, the company makes a wide range of similar engines.
Alternatively, try and find a wrecked VW TDI with a good powerplant, like the one the West Philadelphia high school students used to make a hybrid sports-car (K1 Attack, 0-60 in 4 seconds while burning 50mpg!)
As I suggested in a previous post (not sure if you read them all), please post in evalbum.com, as I'd love to watch your progress!