I currently run only a tach as I havent found a speedo that I really liked yet. My plan is to just make a magnetic pickup speedo some day, but to tell you the truth I havent really needed to do it yet. If you wanted to run a mechanical speedo, you can find one from a Kawasaki 1200 i think it was. It has a 25mm diameter axle, same as the gsxr. I will look it up again as to what bike it was, its just slipping the mind.
EDIT: a mechanical speedo from a Kawasaki zrx should work.