There are chemical changes that occur within the battery when it is at a low state of charg (it becomes less resistant to freeze damage, for one). That's why some batteries are "deep cycle"; they're designed internally to be held at deep discharge without damage, but can't provide the surge current that a car battery needs to be able to to turn a vehicle over. The battery is probably toast. As for the cable, I don't know that you would have been able to get it as an individual piece even back in 1986. The replacement (if that is actually what is necessary, which I doubt) would be to cut the bad section out and replace with a section of at least the same gauge, with a universal battery terminal post clamp.