I have an '85 Briggs horizontally opposed twin with the dual circuit alternator. These things put out just enough to charge the battery and power a couple lights.
I was looking at the wiring though and noticed that the DC is only half rectified with a diode. all I could find is that it should put out 3amps so I'm guessing that's really between 2 and 4 amps and between 12.5v and 15v. Working the numbers it looks like the AC supply must be around 27.6v accounting for a single resistor drop. So if you where to use a bridge rectifier couldn't you get 26.4v 2-4amp output?
Be nice to be able to run an electric PTO.