I think the exhaust will get too hot for an o-ring, so I think I'll just keep it sealed up with RTV silicone. I have to cut a couple new gaskets for the front housing and cambox anyway. The factory ones aren't sealing either which adds to the mess.
Man this engine is perty... The muffler got castor stains within 30 seconds of firing it up for the first time... Dawn Power Dissolver to the rescue!