For rework on this old stuff -- Standard Rosin core, 63-37 solder. Kester 24-6337-0027 or equivalent.
Don't go cheap with some Chinese or Mexican garbage, stick with Kester and you can't go wrong.
I have some Mexican stuff here somewhere... smells like a little kids shitty pants every time I use the stuff.

Lots of diameters to pick from. From tiny 0.015 (and smaller) to large 0.062. Good, generic size is either 0.025 or 0.31". 0.031" is most common.
https://gokimco.com/44-rosin-core-solder-wire-sn63-pb37-031-66.html

If buying from Kimco - get some flux for removing old stuff -- such as this:
https://gokimco.com/circuitworks-cw8200-rosin-flux-dispensing-pen.html