AC charge power is over 50 V, rotor is very clean
The stator voltage is not an indication of the stator CURRENT. Although it usually means things are good, it doesn't always mean that they are.
Measure the stator voltage while connected and it should be charging. If the stator AC is still close to 50V, then the regulator is not turning on for some reason. If the voltage is closed to say 15V AC, then it should be on and sending out current to the battery, but maybe there is not enough current.
Another thing you can do is measure the stator current. If you don't have a large enough ammeter, it is easy to just put a piece of wire in as a shunt. As in, short out the stator with a one foot piece of 18 awg or so wire. Measure the AC voltage drop across that piece of wire. Look up the resistance for that piece of wire and you have the current.