Have you seen these two threads? I did the "free" one a while ago and I'm still very happy with the sound. Seems "hillbilly", but you can't even tell I did anything to mine. Runs great, just like new.
http://www.600cc.org/forum/f89/mod-stock-exhaust-15987/
http://www.600cc.org/forum/f89/free-5-minute-exhaust-mod-19477/
http://www.600cc.org/forum/f89/mod-stock-exhaust-15987/
http://www.600cc.org/forum/f89/free-5-minute-exhaust-mod-19477/