Actually I have flown both engines on a Big Mama from the Airplane Factory which is fairly heavy and could not tell much difference. I lean towrds the Zenoah because it has a real muffler whereas the Mustang 50 has a B&B box collector with two stacks on it, very loud. However I have heard that Fuji made the Mustang 50 and now Tower sells them and the 50 Fuji has a real looking muffler on it, which they might sell separately. Decisions never end do they. The Zenoah is 2.76 cubic inches and the Mustang 50 is 2.84, I'd say calling the Mustang a 50 is a stretch. I think 3.3 cu. in. would be a 50.