Just drill the old rivets out and replace the bumpers.
Be careful not to use too big of a drill so that you make the hole any bigger. I am not certain what size of rivet Corbin uses but I use a 3/16" drill bit to remove most rivets. Once you start drilling them out they generally push up into the seat or you can pull them out with pliers.
Replacements can be purchased for McMaster Carr.
"If you are not handy with your drill motor, please I can oly advise you to take it to someone who does Upholstery."
In addition please be careful that you don't push too hard while drilling those rivets out. when it gives you can puncture a whole in your cover.