Both gears are different depending on direction of rotation..The hole through the body for the wheel spindle is also different sizes for different models..I don't have the information covering all versions so I'm not sure about the actual dimensions...There is also a post war version which is dimensionally different, a bit wider and with a slightly different shaped body...(Made for the M20, not the later type as fitted to T140s, Commandos etc.)....
Not very specific I'm afraid but a few things to research...Ian
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