I've been under the impression that 4 stroke engines get better gas
mileage in aircraft due to their higher BSFC numbers. But I've
recently realized that since the 4 stroke engines are so much heavier
than a same horse powered 2 stroke you end up having to design a
larger aircraft (and engine) to carry the original intended payload
plus the additional weight of the engine. So in the end, a purpose
built 2 stroke airplane will be smaller, lighter may even get better
gas mileage. Unfortunetly, most designs that use 2 stroke engines are
high drag ultra lights so going distances isn't really what they do,
but it would seem to me there is some potential for a low weight, low
drag, in the neighborhood of 500lb empty, 2 seat aircraft designed for
one or two 2-stroke power plants. The Pulsar XP seems close because
I've seen some with Rotax 582's. The aicraft that attempt to exploit
these engines in this way always look like the wings are too small
compared to the proportions we're all used to seeing on C172 and other
similar formula aircraft. Examples are the cri-cri, BD-5, AR-5
http://www.ar-5.com/sportav93.html