Why do Dirt-bikes use 2-strokes (rrrrr-ring-ding-ding-ding-ding) while Harleys use 4-strokes?

Will a dirt-bike make it up the hill with a 4 banger? Yes

Will a Harley cruise down the freeway with a 2-stroke? Yes... (providing you don't run into a group of bikers who immediately skin you alive and use the 2-stroke as your new suppository)

So it all boils down to what kind of power YOU want to put in your plane.