Why is his rectifier out of the question? A broken rectifier could cause the same problem.
If the regulator is good, but the brown wire becomes disconnected, you will have too much voltage, not too low.
Have you checked the stator wires for continuity to ground (while disconnected from the reg. and rec)?
After checking wiring, and charging the battery for a couple hours, the next step would be to let it idle and disconnect the regulator. If the regulator was bad, and pulling down the voltage, it should go up whendisconnected. Be careful to monitor the battery voltage the entire time. The meter should be connected permanently or with very secure alligator clips. You'll have your hands full with other things. If the voltage goes above 14v turn the bike off immediately, and the problem is a bad regulator. If the battery voltage does not go up, rev it a little and see if the voltage ever goes up. If it ever reaches 14, then the regulator was bad.
If the voltage never goes up, then do the same checks but check the voltage at the output of the rectifier, using the rectifier's output for the red meter lead, and the rectifier's ground wire for the black meter lead. If the voltage is good at the rectifier output, then it is a wiring problem.
If the voltage never goes up past 13 or so, then it is most likely the rectifier.
If you want, you can load test the stator. Disconnect the stator from the rest of the bike, then make little jumpers to connect one pair of yellow wires at a time to a spare 55w car headlight (or spare motorcycle headlight). Now connect the meter to the headlight as well. Put it on AC-voltage and let it idle. Revving the bike should easily get it over 13v AC. BUT BE VERY CAREFUL. IT IS VERY EASY TO OVER-REV THE BIKE AND BLOW THE BULB. Make sure it is not a bulb you care about. Do the same thing using different pairs of yellow wires to verify any two yellow wires give good voltage.