My son just picked up a 04 yz450. It has a factory 4 system on it including the power bomb header. The guy he got it from rides mostly desert so he wasn’t concerned with noise or a spark arrestor but personally I feel the bike it too loud. My son wants to go ridding with me and I ride where both spark arrestor and sound db are enforced. Is there something I can do to the exhaust to quiet it down a bit and make it USFS approved?