See this:

[link=http://www.slimlineproducts.com/online_shop/4stroke_exhaust.htm]Click me![/link]


4006S SAITO 120, 150, 180 1 Side Mount Smoke Pitts Muffler with mounting hardware $119.99
4018S SAITO 180 Inverted with Smoke $129.99

Really almost ANY muffler will work on a four stroke engine, including, the muffler that comes with the Saito itself.

The problem is keeping the muffler hot.

MinnFlyer has a great HOWTO article about all of this here.