Translate old php scripts for the site to php 7

There is a self-written cms-system for displaying and editing news. Problems: it is written in shitty (shitty code), uses outdated libraries (mysql_connect, old smarty). It is necessary to transfer (perhaps rewrite) to new rails, replace smarty with twig, mysql with mysqli), embedding the old functionality in the (relatively!) new (also self-written) admin panel.
Offering to rewrite "all on..", switch to Wordpress - not necessary, not welcome. It is required to make a minimum of effort to get the maximum benefit. Though... maybe on wordpress you can, but - then! )
Website: https://www.activequerybuilder.com/