years ago a guy called Pete Lindermere was a well known tuner of these bikes & i think he pretty much always recommended Swarbrick exhausts
http://www.swarbrick-racing.co.uk/prices.html
Stan Stevens is back into it & working on them again himself now he's downsized etc
check some of the engines here
http://www.stanstephens.com/2stroke.htm
here is another oldschool pipe maker that i remember
Jim Lomas
http://www.50cc.nl/2stroke_pleasure....sts.asp~TsMain