what gauge of wires?

exactly. start with the FSM.
go up one size if your setup adds length between the source and the load.
Go up if you've added load. (electric pumps, elctric fans, etc)
Use a wire in the engine bay that is made to handle high heat and oil.
Support the wires and provide strain relief.

Late model cars used SAE 20 ga wire but I would not bother going that small even for gages.
If you are using a newer alternator, use field wires appropriate for the field current draw of that alternator.

I favor Marine wire which comes in AWG gage so has more strands, tinning, and is slightly denser than SAE gage wires.