Stock gauge is generally 8 gauge, but if your repairing it it's better to go to 6 or 4 gauge. The MotoElectric HiCap starter circuit cable upgrades are 4 gauge with decent copper terminals, which is the other weakness of the stock cables.
To be fair though 6 gauge is easier to work with as far as routing the cable goes and is still a good upgrade.
If you want to double check the wire gauge here's a handy chart:
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