I doubt the stator will be at fault (unless it has been damaged mechanically or by a rather sustained and brutal short circuit), but there is still the red wire from the connector to the starter solendoid that could have a problem. I may be unduly cynical, but from the cases I see here at Outdoorking, many second-hand ride-ons come pre-equipped with wiring that has been burned out, ripped out, or revised in a seemingly whimsical manner by a previous tenant. Based on that, I would guess that adulterated wiring is more likely than a failed diode. Even if the diode has failed, there is the question of what made it fail, and what other electrical damage was done by the same series of events. Also, you have not made it clear how you did your voltage test when you tried to verify a rise in system voltage when the engine was running.
Anyway, the tests we've discussed should get you as far as finding both the stator and the diode either damaged or undamaged. It will then be time to talk about other possible problems further downstream.
