I do not know enough about helicopter applications to answer your question. Maybe it has nothing to do with keeping them cool. Whenever I have seen helicopters the engines seem to be revving like crazy. A two stroke will develop far more horsepower than an equivelant size four stroke, but at a much higher RPM.

Maybe the higher revving two strokes, going through the gear train are the only way to get usefull rpm at the rotor.

That is not a statement by me it is a question.

Ed S