BN7 Speedometer issues

Since my '60 BN7 has an unidentified Toyota 5-speed and UK-sourced 3.5 differential gears, I don't have much hope that I can get my speedometer to match the car's road speed. I have a Garmin GPS installed in a mount in the car and use that for reading my speed. Nonetheless, in the interest of fiddling, I have worked on my speedo in an effort to establish some benchmark-reading for the car in the 40 to 60 mph range. I have had the speedo apart a couple of times, and have likewise examined the mechanisms in a couple of other old Smiths speedos. I have gotten my speedo to achieve what a reading in the speed range that I want, but I have now lost entirely the needle rotation when the car is underway. Using the terminology in Anthony Rhodes' treatise on Smiths and Jaeger speedos, I have concluded that somehow my speedo has lost the magnet function. Inserting a section of speedo drive cable into the cable port in the back of the speedo, I get no needle movement when the "magnet wheel" (Rhodes' terminology) is rotated using the piece of speedo cable. It turns freely, but the "drag cup" which holds captive the needle spindle doesn't rotate. As I understand it, the rotation of the magnet wheel driven by the speedo cable causes the large aluminum "drag cup" with the needle spindle to rotate. Since the large "cup" is aluminum, I am assuming the center portion of the magnet wheel, the circular bit surrounding the brass bushing, is magnetized and interacts with what looks like the steel center portion of the aluminum drag cup that encircles the needle spindle. This is a new failure on this speedo. Turning the drive end of the piece of cable inserted in the back of a spare speedo causes both the magnet wheel and the drag cup with attached spindle to turn. I suspect that I have somehow lost the magnet function. If this is so, can it be restored?

Check with Nisonger in New York. They fixed my old speedo on my TD and calibrated it to the rear when i changed the gearing. they also refaced the gauge and put on a new correct needle.