generally, its using an airplane engine in a heli that causes the most problems.

Normally this is attributed to the smaller head heat sinks on the airplane engines, causing them to overheat in helis.

as visioneer1 says, once you get over the physical boundaries (size,muffler,plug,prop washer etc) there are really no other problems, except for the frustration of modifying this and that to get it installed.

I seem to remember another thing, but cant say for sure.... wasn't there a problem with helicopter and aircraft engine interchangeability due to heli engines are orientated horizontally while aircraft engines are orientated vertically? someone here may remember...........Im talking years ago.