What little I've learned from restoring motorcycles and working on cars for 30 years is, manufacturers do not put more than what is needed in their electrical systems, because on a production scale adding $0.10 to the cost of a single wire gets very expensive when they build thousands of them in a month.
In other words, the original wire was probably barely sufficient for the OEM 50-amp current, in addition to being 30 years old and having decayed in that time, you should upgrade it.

1989 Dodge Raider V6
2001 Jeep Wrangler 4.0L
2008 Toyota 4-Runner SR5 4wd