Hi "old man" (name?), electric fan is a good idea, i can use a 5"-8" diameter fan with high CFM, the noise is not a problem.
the added fan might restrict the airflow for the prop, but with proper design and build this can be reduced to minimum and ignored.

talked to a friend of mine that knows his way around these engines and UAVs (he design and build them for a living), he said (like most of you) that there is no problem reversing the engine, the only thing needs to be done is re-time the ignition (different location of the hall sensor or magnet).

any other suggestions regurding engine cooling are appreciate.
will look further into mounting an electric computer fan with shroud, but this will cause additional weight for the fan and battery.