I'd have to have a separate server built with out of date mysql and php software (something no host would want to do due to vulnerabilities) and hope that the database is robust enough to survive the transfer (it was very flakey and had to be locked down to prevent logins and searches due to lock-ups). Balancing that against the number of users who referenced the old site mean it really isn't a viable project.
Some, but certainly not all, pages are cached by Google - e.g. https://webcache.googleusercontent.com/search?q=cache:-0QcgqLBCG8J:https://www.rmweb.co.uk/forum/viewtopic.php%3Ff%3D8%26t%3D2603%26start%3D150+&cd=9&hl=en&ct=clnk&gl=uk
If there's a little green down arrow after a path in Google's results it means there's a cached version.
Try site:rmweb.co.uk/forum em gauge as a search term in Google and you can see how it works