Good luck finding that one in a hardware store. Common problem with that kind of retainment. The aluminum pocket gets punched out and the pads start hanging on the bolt under braking. Are you using aftermarket pads that are too small for the caliper pocket? That little bolt is not supposed to hold the pads against rotation, that’s why it’s snapping.
Drill everything out and use 1/4″ will help a little if you have the meat in the pad tabs. Just make sure it’s a high tensile strength bolt and drill it for a cotter key. You can easily measure the length with the two caliper halfs together. Allow an extra 3/8″ for the nut.
Don’t know why they can’t make a pocket with a replaceable steel piece. Just another example of kart parts that are made to last 3 races.