Gear for mini speedometer

This piece is a speedometer speed adapter for mini Sportpack MPI of the year 1999. I think it should work for all models of mini 1959-2000
Due to a mechanical problem, I changed the differential and the ratio is different. I have a greater acceleration but a lower top speed! I had to change a gear on the rotating speedometer cable, engine side.
It is apparently very hard to find. I tried some without good result. My friend said it was not available. Then I measured my speed with a GPS and c