Yes they can. I've seen it done. I never saw the point, but it's been done. Heli engines do have a bigger head, but they also have their own version of propwash in the form of a cooling fan, so there won't be a problem there. Heli engines are usually more aggressively ported that airplane engines, but it's not a huge difference. Which engine you are talking about will determine whether there's a difference in the porting or not. If you stick with smaller props it will work fine. The difficulty is in the prop shaft. If a prop will go on and it's long enough to get a prop washer, nut and spinner on, then you're good.