That should a easy thing to fix.
Just ftp the virgin files to the site from a install zip.
Then you need to create the config.inc.php file and edit the file for paths dbase etc..(the hard part).
Gizmo posted a generic one here somewhere.
If I find it i will post the link.
Then after that best practice is to go through all the options in the control panel and save even if it looks correct.
One thing you could or will loose is going to be things such as file attachments,avatars, images, extra styles other than stock.
