Pete, I use the Keleo exhaust ring, which fits perfectly and sounds good, the only hole in cowl is for the exhaust exit in the bottom. The flex exhaust extensions by saito will work fine also! If you go to post 943,944 you'll see my engine installation. I also have the 90r running in Hanger 9 Camel, also a perfect match, these engines run good and start on first flip when primed thru the built in priming fitting on carb, no need to choke or use electric statrter.