KSPILOT,

First you'll want to mount the fuel tank as low as you can get it. The fuel line coming out of the tank to the carb should be the same level as the carb, or lower with an inverted installation. This is to help keep from flooding the engine. Next you should mount the engine angled to the left as you are looking at the airplane from the front. This also help lower the flooding problems and allow more room for the exhaust bend. OS makes a 90 degree bend exhaust fitting for the OS .91 FS which fits the Magnum .91 FS. With this and the stock extention and muffler and maybe an exhaust diverter, you should be able to route the exhaust out the lower rear of the cowling. Tower Hobbies sells the extension. Don't have a picture.