The volume of air required to cool any engine is much more than most would believe, let alone the speed it needs to move at. In a hovering situation you will be forced to find a way to pump air to the engine which will present a completely different set of items to over come.

What you are doing is what modeling is all about. It's not always the easy, off the shelf, everyone does this, type of stuff where everyone has a safe haven. Someone always has to led the way and in doing so they have a lot of experimenting and research to do. You may want to look into some high volume electric mini fans. Think computer components with some specialized plenums. Better yet, brushless electric r/c motors turning fans from computer cooling systems.

Any reed valve engine is easy (easiest) to reverse running direction. Most of the piston port engines can as well. As for changing wear patterns, the only place there would be any effects would be at the bearings. Pistons and rings go in and out regardless of rotational direction.