I've been mumbling about doing this for a couple of years but have resisted mainly because my old BMW although inefficient is so much fun .
With our regulations it would have to be a pre 1975 Royal Enfield, which a bit rare but I DO know where one is. Pre 75 and our registration guys don't care what you do.
Using the calculator at top of the page
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com I worked out that I'd need a drag coefficient of around 6 to get sufficient "performance" from the 10 hp
to make it ridable at highway speeds. I reckon 65/70 mph is possible, without a very efficient fairing ,maybe 55?