Bill O'Neal quote.
You are assuming that you should be seeing a spike or an increase in voltage when reving the motor. Normally, you would, except if the battery is fully charged up, it does not need additional voltage, so you are not seeing any higher numbers.
If say, you did see an increase in voltage, it would tell you that the battery is drawing that voltage from the rectifier/regulator and charging system.
If you saw a drop in voltage when you reved it up, it would indicate that the charging system is not functioning correctly and then you might want to test it or install the new rectifier.regulator.
Just because you do not see a rise in voltage is not an indication of a bad charging system if the battery is full and is not drawing any charge
So, with that said, try it, and if something is to go wrong/battery die...in the manual, is a how-to, test for the stator assy...