I am not familiar with the specific model, but I've had a look at the wiring diagram I found online and it looks like a rather standard unit as far as wiring goes.
The first thing I'd do is to check if at constant engine speed voltage just stays down or if it fluctuates: in the latter case the cause is most likely the bridge rectifier, also known as regulator/rectifier. It's usually a cheapish part. Otherwise it's probably the alternator: they are usually big money but they can usually be rewound for a fraction of the price of a new unit.