If you need that replacement breaker you can find it as a RECONDITIONED..... "used" breakers from a number of sites on the web.......just Google Zinsco........The company was litigated out-of-business years ago........
BUT are you sure the main is bad......there are many problems with the zinsco panel.....most surround the inadequate connection to the buss bars.......Maybe it's time to just replace the panel ?
"...measure once.....cut twice....throw that one away and cut a new one...."