Hi, I'm from Belgium and airplane modellisme isn't regulated at all, in fact it is but the regulations are absurd, no one actually knows whitch laws to follow or must be applied, in fact everyone does and will tell you their own story whitch is normally again absurd and propably untrue. Anyway you have to live here to know what I'm talking about to make a long story short.
Most of this has to do with noise ofcourse, so.
Modellers are desperate to keep the noise as low as possible in order to survive whitch leeds to 4 chamber canister mufflers that are more efficient than those of the motorcycles running on the street( often with open exhausts, but nobody cares). Expensive propellers in the hope to gain another db of quitenes, hydro mounts, silent blocks, air-intake filters... and here my imagination stops... not actually.
Airframe vibrations are a major source of noise and we can only see to it that everything is firm and tight but that isn't enough.
50 cc monocilinders represent an important amount of unbalanced mass and torque impulses. Boxers will deal with the unbalanced mass but still remain the torque impulses, so I believe if we could use 4 stroke boxers with unsimultanious ignition that would devide the torque impuls magnitude by 2.. or am I mistaken?
But the problem is that there aren't any on the marked or does anybody know a modern engine of that type anywere around?

To be complete it wouldn't solve the sound problems in Belgium because they are absurd*** as I yet stated, but it would gain another one or 2 db of quiteness I believe. ( I hope)

***Example: modellers club on military airfield is due 72 db at 7 meters while cars on the adjacent street make over 80 at 15 m and
fullscale airplanes a few 100 meters away on the same airfield go way out of the pour audiometer's scale at much more
than 7 meters and that with a much larger
intoxicating area.

Regards