Alas, they're not as fuel-efficient as you think. Remember, those cars have pretty ancient engine technology with carburetors and not really that high compression ratios, and as such you'll be lucky if you can get 33+ mpg unless you drive with a really light foot on the accelerator. The 2006 Honda Civic, on the other hand, has a modern engine with very precise fuel delivery, electronic trottle controls, variable-timing four-valve per cylinder heads, and electronic control of spark timing. As such, 35+ mpg is easily achieveable on steady freeway driving on a much bigger car with way more interior space than the cars you described.
I have owned all the cars I named, and many more.
They were what I drove during the phony "oil crises" of '74.
The Bug-Eye and Fiat 850 got the best mileage, real world, not some contrived EPA estimate.
Whatever shortcoming they may have had verse the modern econo-box's they more than made up for in fun driving.
I filled my tank at least 1/3 less often than anybody else in town.
Currently have a Spitfire as our economy car.