If you copy the ssl and let’s encrypt directories, you have the certs for the existing host name. The server is ready to serve that domain, but dns doesn’t point there, so it can’t.
I usually just copy over the existing yml files, but for this very old site, getting a fresh one is not a bad idea, and in this case, I think I’ve learned that there were some strange modifications that I think will be good to leave behind).
BUT, if you’re clever you can trick your browser into going there and see that things look good, and then you can switch the dns so everyone else sees it. (and hopefully remember to un-trick your browser so that you’re not very confused sometime in the future.)