The 91FX is not a great 3D engine unless you can find a 3 needle carb like the 7L (rare).
Cheap 4-strokes get you less power/weight. Go with Saito or YS and you won't regret spending the money.
Os was in the low power to weight category until the alpha series came out. They are way too spendy though.

Electric is a better option as every day passes. The motors are getting better but the real advances are in battery and ESC's.