The change of grade will not make much difference. You will be able to tolerate a bit larger gap between the magnets and stator for the same conditions.
You could keep the same gap and use a few less turns for the same cut in speed, I doubt that you could get a larger wire size in unless you have several smaller wires in hand.
Other factors come into the equation that makes it difficult to give a definite answer. If you have a short low resistance line to your batteries, any lower resistance may cause additional stalling if you lower the alternator resistance. If you have a long line, the lower resistance will help.
I would be inclined to reduce the number of turns about 5%, keep the same size wire and leave it at that. If it still stalls, open the air gap.
Flux