3 phase vs 1 phase alternator
Rewinding as single phase makes no sense. If you have a 3-phase system it is almost always preferable to single-phase:


the ripple current caused by 3-phase rectifiers is much smaller
the power flow is smooth (vs. single phase which has very high 120Hz power ripple)
use of copper is more efficient


Not to mention that I have no idea how you would manage to rewind as single phase. I think for the time + effort + money you spend on rewinding, you could just buy a 3-phase motor/generator in the voltage you need.