Find the password for user admin from a replicated old version of site
Short answer: it can't be done.

The password is a salted hash, and unless you have access to specialist cracking equipment, you're not likely to reverse engineer that hash in yours, or even your great, great, great, great grandkids' lifetimes. Even with access to the original salt.

As Pierre mentioned in the comments, submitting a password reset link on the live site is probably the simplest course of action. 

Or contact your hosting provider; most will find you a way back into a server you're locked out for a nominal fee, once they've verified your ownership.

Bottom line, you're not likely to recover the password from the hash in the database, so you'll need come up with a creative workaround.