There are baffle inserts available, which are secured in place by a single screw. They are perforated steel tubes. It is best to wrap them with either exhaust grade fiberglass batting, or use stove pipe fiberglass rope wrapped tightly around the length of the baffle (both are secured around the outside of the baffle with stainless steel wire; both are passive silencers, absorbing noise vibrations in the exhaust gasses -- alone, they won't reduce the sound level too terribly much, and they won't have any effect upon the gas flow, no back pressure).
If you introduce baffles, such as stainless steel washers cut in half, welded opposite each other (up/down), spaced several inches apart, the sound will be reduced considerably (the more half-washers in the middle of the perforated baffle, the quieter). This will create slight back pressure, but not enough to require re-jetting. Take the aforementioned baffle, cut slots for the half washers, insert them, weld them, re-insert the baffle into the pipe, and test. You can add half-washers, one pair at a time, insert to see how loud, then stop/proceed with more, as needed.
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Be aware of two things riding a bike with loud pipes: you will likely be in violation of local (virtually everywhere!) sound level laws; you will suffer permanent hearing loss from exposure to noise at this level... Even when riding on a new, nearly silent, Gold Wing, 32 dB ear plugs are recommended just to prevent the wind noise from damaging your hearing! Loud pipes, such as you describe, will add even more damaging noise to your ears.
I have already lost some of my hearing, due to riding motorcycles which had fairly quiet mufflers. I wear ear plugs religiously now, and I would never go without them: I am much more relaxed (loud, constant noise, causes physical stress/tension in your body as you sit astride/ride); I can actually hear approaching traffic better since there is less noise blasting my brain -- I have less to sift through to hear what I need to hear; and best of all, I enjoy my rides much more.
Foam, disposable ear plugs are inexpensive. Custom-made will cost around $120 for a pair, but they are re-usable, and they are comfortable for 6+ hours of continuous use.
Hearing loss is permanent -- you will never get it back. If you think your hearing is perfect, go get it checked. You will be surprised at what the doctor tells you. It only gets worse as you age. Think about it. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII
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