I'd try sealing the gap in the exhaust that you found first. It's easy to find noisier mufflers for bikes but quieter ones might be hard to find. If you're getting tinnitus through earplugs it's possible there's something else causing your problem. I have tinnitus from pistol and rifle competition in the Army and my gun club along with about 5000 hours of light aircraft operation in Alaska. It ain't fun. I wear hearing aids now but they don't do much with the ringing.