I have been working on this bike for a while and finally have it working pretty good, reliable enough to take for 50 mile rides. I don't have a full tank of gas through it since I put the bike computer on it, but my guesstimate is that it will go 150 mpg at an average speed of 20 mph.
Its pretty fun. Not legal yet but I think it could be if I can get a moped sticker for it.
It would be much better with a freewheel hub on the rear wheel, then the motor could be shut down for EOC and pedaling. Right now, if the wheel turns, the motor turns and there is a lot of resistance.
This kit is called a "Dax Titan" and is about $400. It would work well on a recumbent with a fairing. Top speed is 30mph but on my bike 20mph is more reasonable, it gets a little bumpy at 30. A front suspension fork would be a good thing. It is 50cc and about 1.5 horsepower.