This may sound a little silly, but - IIRC isn't the empeg fused on that wire near the chassis? If so, take a look at what rating it has on it. I'd put another one of the same amount next to the battery on the new wire, whatever that happens to be. And for the wire, "about the same or a hair bigger" than the current lead should do it.

I know, real vague, but if nothing else it could give you a warm fuzzy about what you use.