There are no aftermarket exhaust options available.
So, if you want a different exhaust note or look you only have a couple of options.
1. You can drill holes in the rear plate on your muffler.
2. You can install an aftermarket universal muffler.
3. You can have something custom made.
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With any significant exhaust change you will be leaning out your fuel/air mixture. So it will be likely that you will need to rejet your carb accordingly, if you want your bike to run properly.