how can i modify my stock exhaust on my crossfire150r without buying a new one

Pud will chime in I'm sure. He has a good method

Here is a how-to from NEBA that tells how to gut one. Haven't done it my self so I not sure but read it if you would like to.

http://www.northeastbuggy.com/engine_mods.htm#Gutting%20Your%20Exhaust

here is a easy mod to do to your exhaust if your on a budget and haven't a lot of tools to work with.
i drilled mine out some to get a better flow and leave me with good bottom end power.
i just went to tractor supply and bought a 12'' long 3/8 drill bit.i still think you can buy the bit for less then 15 bucks.
my exhaust had a tip that came out and turned down.i took a hacksaw cut tip off flush with exhaust.
each muffler might be a little different in side,so you might have to do a little feeling around with a piece of wire before you drill.
the red box's i mark that are drilled just stop there do to the baffle and things in it.the other 4 holes marked in yellow go through muffler then into a plate that holds the baffle and things,the plate is a inch or two in once your through muffler.there is also a back plate in muffler that is why i used the long bit that let me drill that to.the reason the 4 holes are there was to let me get a nice straight drill without hitting the stuff inside.and you can see that behind the red boxs if you look.
pretty easy mod i think,if your interested in doing it that way.
my muffler don't sound bad not to loud and for me i didn't want to lose all my backpressure for bottomend power.

pud