Not really sure what you are asking, but in the picture you have provided, if the engine were mounted in the rear or the pod you would need either a reverse pitch prop or for the engine to run backwards. In either case, the air coming through the prop needs to be directed toward the tail of the plane, pushing the plane forward. The way it is now, mounted in front of the pod, it has a propeller that is pulling the air through and directing it toward the rear of the plane. If you were to turn that same engine around and mount it on the rear of the engine pod, the easiest way to move the plane forward is to buy a reverse pitch prop and keep the engine running in the same direction it currently is.