one way you could do it is cut a biturbo about 4 inches in front of the exspansion chamber and make 2 more cuts on the end of the pipe you cut off 180 degrees from each other. the you have to put it onto the header pipe that is on the supersport then you have to figure out where the bracket to hold your biturbo on has to be welded. then you must take the back of the biturbo and the back of the stock muffler and have the bracket cut from the stock and welded onto the biturbo. all and all this for say 2-3 miles an hour more is pretty expensive but it will sound cooler.
another thing you could do is order the performence kit from polini for the garelli vip which has the same cylinder as avanti. i think its 65cc and comes with a larger inlet pipe. there may be a perf muffler there too but i forget