Advice on importing generated html files into a Robohelp 2017 project

I'm looking for advice on how to resolve an issue where a Robohelp 2017 project has been lost and the only backup is from over a year ago, not good I know.

There is a responsive html generated version of the helpfiles from not that long ago.  Was wondering if anyone had any advice on how to handle this, although I know it's not going to be a simple easy fix.

And to throw things into the mix, I want to upgrade this project to Robohelp 2020 at some point.  So I have a few questions and would be grateful for any advice.

• Is it best to upgrade to 2020 then sort out the problems
• Is importing the generated helpfiles into the project going to be easier than copying and pasting their contents? There was about 800 topics in the project.
• Is there a way of importing the images that are in those html files?

I'm not that familiar with Robohelp 2017, as I haven't used it for years.  I use Robohelp 2020.  It's a colleague who is using that version.