I guess you could install a bunch of old versions on Ruby, gems, etc. on your machine too, but docker, or a VM might be easier.
Or you could go through, remember how all the code works and upgrade it to the latest versions of everything to run it. Assuming the SSG du jour 2013 is still maintained :-).
I tried to resurrect a 2 year old NodeJS project and gave up trying to upgrade it to the latest versions, NodeJS people really love their major version changes, and breaking back compat for no real reason.
A SSG generated site will be the same in 10 years as its today, even if you might need to fix some things with your project to make further updates.
With wordpress you could export it to HTML as a backup but it is an extra step.