On Honda's website the gx31 is no longer made it's been replaced by a gx35 it weighs around that same 7.5pounds with the fuel tank. The engines are rather small I've seen them at local small engine shops...the issue I'd see would be clutch hookup...it's alotta torque compared to a 2 cycle the stock clutch bells would probably just break real easy. On planes it's kinda different your not looking for high rpm just mostly efficient thrust, so bigger prop used more thrust available....4 strokes can easily swing larger props where 2 strokes struggle. On a car same could be said with tire sizes....the 4 stroke would easily haul around larger tires where a 2 stroke would struggle alot more...as far as tuned pipes go those aren't needed on a 4 stroke for power gain....possiblity with a 4stroke could be turbo charging or running alcohol based fuels for more power...there's alot of advantages to them and disadvantages...NOS would be more effective via a 4stroke engine...turbocharging is possible on a 4 stroke...fuel injection also is possible....but in the motorcycle world a 4 stroke dirt bike doesn't even come close to the performance of a 2 stroke dirt bike...mainly due to the 2 strokes reed valve fuel delivery assembly and expansion chamber allowing the 2 stroke to produce outrageous rpm's and have that extra snap we know as a powerband...but then again a turbo charged 4 stroke would really scream
I think if they could be lightened up alot more like the plane engines and come in around that 4 pound range it'd be nifty but at 7.5 pounds somethings got to compensate for the additional weight....their cheap engines around $200 guess it'd pay to pick one up and tinker whats the harm in tinkering...worst case ya can toss it on a airplane or scooter