Disconnect the single wire from the regulator and make sure that you are getting battery voltage from the wire coming from the tractor. If you have voltage there then disconnect the two wire plug going to the regulator and then start the engine and run at full throttle and connect your meter between the two pins and check the voltage, being aware that the meter needs to be set to AC. the voltage should be 28 v or above. If those two test check out ok, then replace the regulator.
If the voltage from the Stator is low then replace the stator under the flywheel. If low voltage from mower battery, then determine where the problem is.