The threaded inserts are called riv-nuts. There are lots of riv-nut kits and individual parts on Amazon (just oneexample) https://www.amazon.com/Riveter-Riveting-Setting-totally-mandrels/dp/B00R1FYLWU/ref=sr_1_4?keywords=rivnut&qid=1582325361&s=hi&sr=1-4

You'll have to determine the screw size from one of the existing good screws and then purchase the right size insert. Then use a drill with a bit that is just barely bigger than the hole to drill out the threads of the damaged one. Work your way up in bit size until you just drill away the entire insert without enlarging the hole in the frame tube. It's easier than it sounds.

Once you've drilled out the old insert you can replace it with a new one. The only drag is you may end up with remnants of the old insert rattling inside the frame tube if there isn't an opening to get them out. Small annoyance.

I use a Quickie Ti that I've had for over 20 years and have replaced almost all the inserts by this point. Everything else has held up great with just general maintenance and replacing wear parts and I couldn't imagine using a different chair so a riv-nut kit is useful to have.