The ones I'm familiar with drive a toothed wheel from the front of the sprocket. The angle geared speedo drive itself actually mounts to the lower frame leg with a clamp and is adjusted to line up with the sprocket. What year are you working on?
I have a 1940 Scout Junior 30/50 I know they did not come with speedo's or they were optional but I have seen photo's of a rear sprocket drive on bracket with Corbin speedo. I believe I have a 38 tooth drive cog on the back hub but not sure Number of teeth required for the speedo cable sprocket . How many teeth the rear sprocket is suppose to be? How long will the cable be? All these questions as well as the speedo is built for Kilometers.