randelator, correct me if i am wrong.. but you have to stop the motor then have the motor itself turn backwards... and reed two-strokes are ideal for this, ... or so i was told.
from wiki:
"In common with many two-strokes, reed valve engines can rotate in either direction. This has been used to back up microcars such as the Messerschmitt KR200 that lacked reverse gearing, and it allows flexibility to pull or push model airplanes with either sense pitch propellers.
Many early two-stroke engines, particularly small marine types, employed a poppet type check valve for the same purpose, but the inertia of the valve made it suitable for low speed use only."