The little 10 hp diesels that are easy to fit to the Enfield are really 9 hp so using 75% power (6.5 hp) gives a cruising speed of 65 mph. That is plugging in a cd of .35 for a Craig style fairing as you suggest. That would be entirely adequate for my use and allow freeway travel.
I'm not allowing for headwinds, much more than 10 mph would see me on a side road but still OK. The calculator gives the mpg as US 126 at 65 mph. If it's biodiesel I only pay for 25% of it, it's cheap!
Making of the 10HP Diesel Prime Mover - ADVrider
Looking good !