Do be warned that the 135 degree drive speedos come in a wide variety of gearing options. You can often pick one up cheaply enough at autojumbles but most are for cars and have a flex speed of over 3000rpm, flex speed being the number of rotations of the speedo cable per mile travelled. Bikes tend to have lower flex speeds and I think the one for your bikes will be 2080. its logical enough, given that most cars in the 1930s probably travelled at about 40mph max whereas the hooligans on bikes were doing 60mph, meaning that the rotation of the speedo mechanism was about the same. You can only be sure of the gearing by dismantling the speedo and counting the teeth on the odometer drive gears - which is not possible at the autojumble! Speedos geared for bikes are quite rare but converting the car ones to read the correct speed is not too difficult. Getting them to read the correct mileage means finding the correct gears, which means finding a bike speedo.......
The bicycle speedo on the handlebars is definitely the easiest, cheapest and most accurate solution.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.