The main issue with replacing the stator wires is to reduce the electrical heating from the 30+ amp current they have to carry. A different aspect of this is that any in-line connector becomes a hot spot because, as corrosion occurs, that causes the connector resistance to increase and the heat generated to increase in this region. This damages the connector (melts the plastic ones) and overheats a few inches of wire both sides of the connector.
But that’s not the whole issue. Any wire, no matter what size, has resistance (so many ohms per foot) so any wire will heat up. Obviously the larger the wire, the less resistance it will have to current flow, and the less heating that will occur for a given current.
The type of insulation on the wire is also important to this discussion. Different materials have different temperature allowables for continuous operation. For example for 10 gauge (AWG) wire in 30ºC (86ºF) free air:
55A will heat high density polypropylene to 90ºC allowable
58A will heat Polyvinyl Chloride (PVC) insulation to 105ºC allowable
75A will heat Kapton, Teflon, and Silicone insulation to 200ºC allowable
So, silicone insulated wire should be used for higher current ratings or hotter operating environments.
But to answer your question ...
If you're relocating your regulator to behind the license plate (which I strongly recommend) then the wires are routed outside (the hot environment inside the fairing) so use AWG 12 gauge PVC-insulated wire.
If you're keeping the stock regulator location, use AWG 10 gauge. Use Kapton, Teflon, or silicone insulated wire if you can find it (McMaster-Carr), but PVC-coated wire from Radio Shack will work, at least for a good while.
