Easy.
Just add includes with a variables using PHP (or other coding type).
Code:
<title><? print($pagetitle); ?></title>
on main page (that should now end in .php or other language)
Code:
<?php
$pagetitle='home';
$metaDesc='Welcome to the homepage of your site';
?>
Becuase the site is massive you can use Dreamweaver.
Then... Edit > Find and Replace > Drop down 'Entire Current Local Site'
Make changes carefully bit by bit and replace all.
Now test it many times. Get some one who owns the site to test it.
Oh and BACK UP THE OLD SITE!!!!!