Check the specs on your OEM battery cable. Big doesn't mean it has much wire in it. Recently cut into an old one....lots of rubber sheating, not much copper, about 10Ga to my eye.
BTW that type of battery you are contemplating with the wires built in, often used in old 6V BMW motorcycles.
Fuse it to be on the safe side, altough if you think you may blow the fuse troubleshooting, I'd use something like that:
Thread worthless without pics....I want to see the old scooter.