Speedo Cable blues!
Hi MaxGlide, well i don't know if Whizzer parts will work on your speedo, even tho it looks like Stewart Warner on the backside, BUT I do have a solution/suggestion for you.
Some bikeshops sell a plastic speedo complete for 13-20 bucks with the cable and little plastic drive sector, these are the same fittings used on the Modern Whizzer, and many Stewart Warner clone type speedos.
I would take my speedo to a shop, try their cable, then buy it if it fit, and use the drive and cable, then knowing that standard Stewart Warner parts will work on your unit.
Good Luck,
Mike