Use SWG 12 gauge.
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 for example, silicone insulated wire should be used for higher current ratings or hotter operating environments.
The environment that the wire sees is important, and this is where Ducati engineers screwed-up on the superbikes. The above temperature ratings assume that the wires are located in 30ºC free air. The stator wires run first internally to the engine, and then are enclosed on a sheath that passes over the engine and internal to the fairing. So the insulation on these wires don’t see adequate cooling, they exceed their performance rating, and loose their insulation properties.
The portion of the stator wiring that run in the sheath is where insulation failure is critical. The stator wire are held closely together and insulation breakdown from heat causes them to short together at the higher output voltages at higher engine rpm.
The leads attached to the regulator/rectifier are a smaller gauge but they also see a cooler environment than the sheathed wires. Sure, they’ll be heated more than the larger gauge replacement wires but they have better cooling. So, just make a low resistance splice to the RR wires.