+1 Glort.
Tanman you could spend a small fortune on engine management systems, exhaust emission sensors and the like and probably get an improvement in efficiency of around 15%, with a much less reliable engine due to the fancy electronics. You could send the head away to have it ported and fit a turbocharger, might get you another 5% efficiency and more output from the engine. If you are going to run waste oils, which cost you next to nothing, who cares? You are still much less of an environmental vandal than you would be burning regular diesel from the pumps.
If you need an engine to produce 10 horse power buy one that will do 12 or 15 and it should last you a lifetime, with regular maintenance.
Bob