First post here, so please help me out if I'm leaving out essential information or violating some local standard.

EDIT to add: This question is really meant to be specific to vintage Schwinn's, if they used standard outer cables, I would just do that.

I'm fixing up (really just getting back into service) a Schwinn tandem from 1968. I want to replace the shifter cable, but wonder whether it is usually better to replace or reuse the outer cables? From what I have found browsing, they are not standard, and so harder to find the proper replacements. The ones on the bike look like they are in decent condition, just dirty.

What do you all do, and what is the standard for deciding to replace?