Try AOMCC single spares re speedo gears, Peter Kemp has them listed on the spreadsheet.
4036-29 Draganfly has them listed as excellent stock level
Regarding bottom of headlamp retaining clip height to the central ridge on mudguard: scaling off an original WNG photo imported into AutoCad I got 14cm based on 45.5cm centre of wheel spindle to centre of lower front girder fork spindle (45.5 measured on my WNG). However, it is possible for the mudguard to be slightly higher or lower depending on its mounting bracket/position. A better measurement is the vertical
height from the centre of the front wheel spindle to a horizontal line projected out from the bottom of the headlamp clip'''''' based on the scale photo this is is 52cm, (mine measures 51cm).
Which speedo mounting do you have? side or central. From the parts lists it appears that the side mounted ones were used from the end of September 1941 (from engine number 10789/frame 28241) (central mounting upto this point)
If it is side mounted then one bracket for the speedo comes from the headlamp mounting bolt so the headlamp needs to be the correct height or the speedo cable will be too short/long. The cable for the side mounted is 25 inches long (shorter than the central one which is 28.5 inches long) See this post I did on the issue viewtopic.php?f=49&t=9505&start=20#p69974
Hope this helps