Creating new web site based on old CI version

The company I work for handed me a copy of a site that was built on CI version 2.1.3. They want me to use all that functionality and create a new site for a different arm of the company. As a quick check, I built a local server using all the up-to-date versions of everything, copied that site including CI and it failed miserably as expected though I think much of that was due to using the latest version of PHP.

So I installed CI version 3 and copied all the directories from the old site just to see what would happen. While CI3 comes up just fine on its own, putting the old site in there has issues with pages calling other pages which want mySQL access, jQuery animation and on and on. I just don't know how deep this rabbit hole is going to go.

So my question is, is it crazy to think I can take a site built on version 2.1.3 and rebuild a different company site using current versions of PHP, mySQL, etc? I'm thinking I need to tell them we need to build from new but if someone has a magic formula, I'd love to hear it.