Here's a test for the starter/generator and regulator...
Find the green wire from the starter to the regulator. Unplug the plug in the middle of the green wire.
Connect your voltmeter to the battery.
With the engine running at governed speed:
Hold the green wire FROM THE STARTER to frame or engine ground.
The voltage should jump to around 18 volts. If it does go to around 18 volts, the regulator(or regulator connection) is bad.
If the voltage stays low, check the green wire for any breaks...