The most obvious way is to measure and copy the thickness of the copper core.
I'd use a vernier caliper with a dial indicator for ease of use, but any accurate measuring device will do.
Of course for the Siba black box, they use different gauges for the various wires according to the in service amp load that each is expected to carry.
I believe that the wire that energizes the starter circuit is the heaviest.
Next highest gauge being for routing the Dynastarter's generated charge to the battery, etc.
Do you not have original wires to copy?
Even burnt up wiring leaves a measurable core.
What exactly is it that you need to rewire?
