I changed my sprocket and put a Technigas pipe on my sprint. I heard that the technigas is the all around best pipe out there for the A55 right now. you might find the biturbo a bit cheaper though.
http://www.mopedshop.com/Moped-Exhaust-Systems-s/36.htm - bottom of the page is the pipe, cheapest i could find it.
https://www.mopedarmy.com/forums/discuss/7/63858/63858/ this link here says it should fit.