Alternator stator is under the flywheel. If tractor has electric PTO clutch you will have one of the higher output alternators with a regulator/rectifier assy. You should be able to unplug the stator from the regulator. Measure the stator for continuity with the engine NOT running. If you have continuity, start the engine, run to high idle and measure the AC from the stator. It should be around 40 VAC. Also check that there is 12 volts on the red wire to the regulator. Some, if not 12 volts on the wire with the ignition switch on but the engine not running, the regulator will not work. You could have a bad switch or a wiring problem. If all of the above check out, replace the regulator.