mpg, not mph. i was looking for efficient American made transportation since gas prices are probably going to be shooting close to double digits soon. i was thinking that reducing the fuel intake and sticking on a turbo (please realize that i know nothing about this sort of stuff) might work. but that would be expensive i'd think. i don't care about top speed at all. i was actually hoping to put some outrigger wheels on it, have a custom cabin of sorts built for it to protect from rain and lightening and have a trailer hitch attached so that i could do thing like buy groceries. i didn't want to mention that as it probably sounds a bit outlandish.
You could possibly rig it up to be pedal driven till you get to 20 mph then hypermile it once you start it such as shut it off going down hill and get up to speed very slowly. May start with a Buell Blast.
Adding weight with an enclosure and trailer will kill your gas mileage. Take everything off that's not essential for making the bike run. Belt guards, heat shields, fenders, speedometer, one mirror, your socks, turn signals, passenger pegs, etc. Go on a diet. You'll increase mpg, but I don't know about getting a 50% increase.