The main differences are the carb and the cooling fins.

Heli engines have a carb thats optimized for mid range where we spend most of our time, airplane carbs are setup for top end.

The head also has the cooling fins. Some engines, not sure about the 50, use a shorter crank shaft and you would also need to find the thrust plate and woodruf key which sometimes find a way to disappear when your using the engine in a helicopter.