Lots of alternators, probably most, will make 120VAC.
Unless you mean 120VAC 60HZ single phase, then you are out of luck without a lot more research. And the more you research it, the more you will find you are pretty much out of luck.
Most decent DC generators are actually 3-ph AC alternators, intended to be rectified to DC output.
So far the only thing we know is it is hydro, without enough water sometimes, 250' or 600' (?), and 12V batteries.
Without the head, flow, type of water wheel, etc, etc, etc,
nobody can point you toward anything relevant.
G-