linkpin vw front beam for low maintenance and ninja 250 engine and swingarm.
Kawasaki just released the "updated" version of the Ninja 250 this year so older models go for way cheap. The engine is very simple and makes 36hp and 17 lbs plus it has a 14,000 redline
I have a 2005 Ninja 250 that I ride the crap out of and the worst mileage I ever got was 45 mpg, and i'm talking about practically redlining it in every gear, lol. SuperSuperfun bike though, and i'm a 200 lbs guy
In a chassis with better aerodynamics (motorcycles are horrible) I would think 100+ mpg is possible with a few mods like a sprag clutch. That would allow you to Pulse and Glide super easy because the trike would freewheel anytime you got off the throttle.
This is exactly what I plan to start building in the next month. I was able to score a complete low mile 1995 250 for $200 and I have a few front beams to choose from in my backyard. Should take only about a month and less then a total of $1000 to complete.
Sure a scooter engine would work but 14,000 rpms of 250cc fury sound AMAZING