Other ways to quiet the bicycle motor kit are to-

Place rubber blocks between the cooling fins on the head and cylinder.
If you experiment with a better air filter/box you could quiet any two stroke nicely.
Make sure you have proper lube and tension on your drive chain-3/4".
Secure all "sheet metal" such as fenders, chain covers, reflectors, seat springs, or anything else that can vibrate and make noise.

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If it ain't broke, and you mess with it long enough, it will be.