Anyone know how to test a volatge regulator on a Kubota T1600, please?

Hi just acquired a T1600 which won't stop or charge the battery. I've tested the output from the generator which is about 22v A/C which seems to be correct and fine, the ignition light used to be bright and remain on, its now occasional and very dull, even when engine is not running. I've put a meter across the battery, voltage is about 12.6V DC, and this does not change when engine runs, so I'm guessing the regulator may be faulty. Anyone know which wire goes to what and how to test it, please.