You need the bisson or slimline pitts muffler stated in the manual. Mine is a beautiful installation where only the pipes come out at the rear of the cowl. I also have an on board glow driver so no glow-plug hole, only needle valve and pipe holes. Use a dremel tool and paper templates taped to the fuse to find hole locations.

Yak

Opps, I just reread that you have a 4-stroke...no way around bigger holes...sorry.