find someone to cut it in half remove the rattly pieces and weld it back together. as long as it still has the end cap it shouldnt be that much louder. i would try to keep the spark arrestor for riding in the state run parks.
hopes this helps
edit: i forgot to mention with this idea you would still probably have to rejet. you may get away with raising the needle on the slide and opening up the pilot screw a little bit. any change you make to the back pressure will likely cause you tou readjust your carb. the main jet basically affects the fuel mixture at 3/4 to wide open throttle. it isnt adjustable therfore has to be changed to allow more gas in mixture or subtract gas depending on the exhaust you choose.