Searching old and new parts of site

I've just made the big change, making the WP site the active part of the site, so that if you just type in the domain, you go to the WP pages. Whew! Still a little slow, and I have an urgent request in to the ISP guys to figure out why that is.

Anyway, one thing I haven't figured out (among many!) is how to get the search function to reach backward and search the old HTML files, not just within the WP database. Those old files were searched in the past with a Google custom search. Should I just put that in place in lieu of the built-in WP search widget?

Can I put in some code to force the WP search to cover the whole site, old and new?