really if you got an air cooled bug powered kit car it would probably get 35mpg as is. Add fuel injection to it with a wideband o2 and it would probably hit 45-50mpg.
Go to a 'half bug' engine and pick up even more mileage
I think to mod a kit car to get good mileage the bug engine is as good as any engine to mod so you might as well stick with it. Adding a computer to a carb engine is really a lot easier than you think it would be so that would get rid of the biggest problem those engines had.