Moped Army

A Biturbo is already a speed pipe upgrade. There are slightly faster ones available but for a lot more cash not worth the couple of mph yeild extra. The stock carb can give you over 40 mph (I have an A35 that does 47 with a stock 14/12) The stock airbox is pretty restrictive, I run no airbox at all. As far jets go no one can tell you what is the exact number for your engine/pipe combo. You have to just experiment. Usually the stock jet is close if you have a biturbo and no airbox. You may have to richen a tad (one or two numbers higher). Do a plug chop to see if you need to change jets. If you need to richen, a cheap way out is to use jet files to open the jet until it is the necessary diameter.
Many people will say you need a bigger jet with the expansion chamber type pipe, but my experience has been the opposite. The bigger pipe sometimes pushes unburned fuel back into the cylinder, making it run richer. By pulling off the airbox I have leaned it out to the right mixture without messing with the jets. Remember this is not an exact science, there are too many variables. Trial and error is the only foolproof method.