yet at several posts OP says battery charges up fine, runs the starter fine and no apparent "voltage change" when hooked up to a different battery (lawn tractor battery)
well in theory with RPMs current/volts should flow out of the stator/rectifier and to eliminate the battery " option" as the source or the cause a simple 12v filament bulb could be attached to the output of the rectifier
alone and grounded on the engine frame may solve the mystery
IF the rectifier/stator assembly is pushing out volts and amps the bulb should light up.If it does not the problem is engine side. if it does problem is battery side but all his batteries can't be bad can they ?
An ammeter would do the same to observe things between rectifier and battery.
the ground hog is out again...snow is all gone