DIAGNOSIS Probably not the alternator (stator) itself. Low voltage almost always means the engine is running way too slow, so the RPM is low. This is either a massive overload (unlikely to cause 90V) or, more probably, the governor spring slipped or the throttle isn't opening.
FIX First, check the obvious: make sure the Eco-Throttle switch is OFF. Eco mode lowers RPM under light load, but a failure here can keep it stuck at idle speed. With Eco OFF and no load, use a multimeter to check voltage. It should be 120-130V. If it's still low, you need to check engine speed. The only reliable way is with a handheld digital tachometer aimed at the flywheel nut or by measuring frequency (60 Hz = 3600 RPM).
IF THAT DIDN'T WORK If the engine is indeed running at full 3600 RPM but voltage is low, then you might have a failed AVR (Automatic Voltage Regulator). This is a small circuit board usually mounted near the brush assembly. Replacing it is a common fix.
HEADS UP Never adjust the governor to try and raise voltage without measuring RPM/frequency. Overspeeding the engine (above 62 Hz) can damage it and the connected appliances.