Motors came be 99% efficient, compared to ICE at 20%. Cooling isn't that huge of a deal. If a wing can operate at a given altitude you can design electric propulsion that will also work there. The U2 operated on a turbine at 70,000ft. A turbine is just a very complicated prop at the end of the day so in theory a large cord/diameter prop or a high speed ducted fan would also work. The actual altitudes were're talking about for GA are far lower though. Most GA aircraft top out below 18,000. Electrics will likely prefer operating at 20,000+.