I suggest you work with Weldangrind.
He will tell you how to eliminate the catalytic converter, and can fix you up with a muffler.
Gaining more torque for climbing hills is simply a matter of choosing the correct sprocket combination. However, if your bike is typical, I am betting your current sprocket combination is ideally suited for climbing hills.
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Spud
"Never argue with an idiot. He will drag you down to his level, and beat you with experience."
Mark Twain
2015 Zongshen ZS250GY-3 (RX3)
2006 Zongshen ZS200GY-2 (Sierra 200)
2005 Honda XR650L
2004 Honda CRF250X
1998 Kawasaki KDX220
Mods made to my Zongshen ZS200GY-2:
http://www.chinariders.net/showthread.php?t=6894