I have one of those Yanmar clones, too. They are popular for motorcycle diesel conversions, especially in the Royal Enfield Bullet (because the Bullet has a separate transmission case). I've read reports of 160 mpg in that application.
OTOH, folks who use that engine with a Comet CVT only seem to get around 100 mpg; which my '74 XL70 with a Chinese 125 clone engine beats handily.
That's a great idea, using the trans from a DT200. I've considered doing something similar with a Honda CB500T.
That's interesting about the drone engine, too. About ten years ago a company called 2SI was doing similar things with a man-portable 2 stroke generator for Army Special Forces.
Speaking of military gear, are you aware of the Kawasaki KLR diesel bike that was built for USMC?