To reverse the direction you have to reverse the relationship of the power applied to the armature (spinning part) vs the stator (fixed part). Since the fixed part is usually an electrical magnetic (soft iron core with the "field" wire wrapped around it), you need to reverse the N S pole relationship:
Did you ever read up on installing a generator on an engine? They tell you how to, but in short the task is to flash the stator with the right magnetic polarity to put out the right polarity voltage for your system.
What I would do is to make your mechanical installation, but before you make the electrical hookup, google "flashing a generator" and in reading what they say, just reverse the leads. Or post the question on here and what ever they say, just reverse the wires you connect to where to do the flashing.
If they say connect the pos to terminal x (whatever it is), connect the neg there and reverse the other terminal likewise. Remember this is a "flashing" which means just run the wires across the terminals which will spark and shoot current through the stator windings but that will polarize the iron core which lies inside the outer case.
Then hook it up normally except for the output wire which you want to check the voltage polarity before you hook it to your regulator. Crank her up, take your reading, if good, shut her down, connect your output lead and you're good to go.
That's what I'd do.