I think you're going to have to compromise quite a bit. You can't have extremely
light weight, extremely great crash protection, and extremely great performance
all at the same time.
I hope some of the ideas others have posted in this thread are "sinking in" . . .
Regarding good fuel mileage, you could research what Doug Malewicki did. He
has a nice website that describes/shows a street-legal motorcycle that was
demonstrated in California quite a while ago. Its fuel economy was quite
impressive. Yeah, there were also some compromises. For example, no
"jackrabbit" starts. Doug used to sell plans for building a similar vehicle. I
don't know whether he still does, though.
And by the way, I used to have a (totally stock, unmodified in any way) Honda 50
that got about 200 miles/gallon. It wasn't very fast, and had no heavy roll cage.