Is there an easy way to revert back to original untouched sage files?

I was modifying a bunch of files and really want a new clean install. Should I just delete the files under my theme and reinstall? Is that ok to do?

There’s no problem starting again with a fresh install - if you give it the same name as your original theme, WordPress won’t even notice the switch.

I’d suggest you keep a copy of the original theme somewhere until you’re sure there’s nothing you need to refer back to or copy across…