A magnum 91 would be ok if thats what you have, just remember the heavier the engine the more weight your gonna have to add to the tail to balance correctly, resulting in an overall heavier plane. Keeping the plane light is beneficial. The lightest 4 stroke is the Saito 91 weighs, 19.6 ounces, and a 2 stroke 46 is average around 17 ounces. Try to keep the engine weight between these weights, or not much more. Daz...