As Ian said the spiral gears are cut the opposite way to most of those type speedo drives, which makes the M20 drive so unique and hard to find. The only other similar drive that I have found was from a post war Flying Flea which has a much smaller spindle hole which can be bored out.
So if you were to use the correct body from say a Matchless or something and install spirals that suit an M20 left side rotation, it would work. Ron
