To answer your question. No you do not an engine box for cooling. It was devised as a way to allow various engines to be used on a single design. Miles designed that plane for the avaiable engines, at the time. If youe engine fits the cowling mounted on the firewall, great. Some builders move the firewall to accomodatre engine of choice. Stand offs and boxes can also be used if you need to extend the engine out to match the cowl. Just make sure it as strong and secure.