i run a kazuma sometimes,(running pro1 now,its easy for me to switch)what i used to do with the kazuma when i'm not racing to curb the noise:
was get the silencer(4.5") get another silencer,that was smaller (4") and have it welded onto the back of the first.
it is now longer with two flanges.i then take regular fiber glass wrap (the ones you use to stuff/line speaker boxes (insulation) and wrap in between the two flages back and front until it came up to the exact hight of the flanges.i used a single hose clamp (the one that comes with Intakes
and clamp around the middle to hold the fiber glass wrap in place.its a lot more quiet with the silencer alone and you do still feel some gains,when i'm ready to race i take it out.
the kazuma has a sharp bend clase to the back,you have to measure accuratly to put the second flange on so that both fit into the exhaust without the back of the pipe getting jammed on the inside.