Looks a nice machine and it should work well. Your alternator is very close to one I built for use with a buck converter, it should do very well in low winds but you may have stall in the higher winds.

Try increasing air gap first and see if you can raise the cut in speed as far as possible without too much loss of low wind performance. When you have got that optimum then you may need to add a bit of resistance in the line but at 12v it may not need it.

Let us know the results.