If you’re talking about the steer wheels and wanted to replace the “tires” DIY, you might research about molding new “tires” onto the old wheels with polyurethane elastomer of appropriate properties - similar durometer, etc. to what original rubber was. You’d have to make a simple mold and call a supplier for advice on the resin (get whatever the smallest kit is). If you do it, get a bonding agent for the polyurethane to metal for the wheels and of course make sure they are prepped with a light sanding, whatever, and clean. If you have time and replacement wheels are that far from reasonable, sounds like a cool project.
The issue is the pneumatic kit is around $40. The wheels are about the same. So $200 to get my pallet back to functional or spend some money and buy a new 1 for about the same and probably has a 1 year warranty.