I have no idea what a magnetic alternator is. You can test a normal alternator a volt meter. When the tractor is running at about mid throttle the volts should read about 13.8 - 14.2. Right after starting it can read up to 15 or so volts for a short time. If you don't have a volt meter then hold a metal screwdriver or similar to the rear bearing on the alternator. If the bearing is magnetized then the alternator is charging. More magnetizm equals higher volts.