Had an issue with the speedo in my '40 acting up. Would work, then not, readings erratic. Turned out to be a worn out cable. First thing I would check would be the cable for proper length and engagement at trans. & head. Also, watch for kinks or tight bends.
I also pulled apart the head to check gears, lubricate, and install new decals on face and odometer wheels. Not a hard job. Just make sure you get things back in the original positions.
