Maintaining an old Sage 7.x site: complete rebuild vs update?

I’ve inherited a site that needs updating, built on Roots/Sage 7.0.3 with LESS. I’ve only got the published files, none of the node_modules or other build files. What’s the most sensible way to proceed. Do I need to completely rebuild the theme on the current version of Sage, or is there a better less laborious way?

If you have the CSS and JS sources then the path of least resistance would be to keep an untouched backup and see if you can npm install then run the build.