I have this all the time restoring golden oldies. Fortunately you've a good brake to play with. Get a pair of needle nosed pliers and unhook the springs, undo everything then take the bolt out and along to a D.I.Y. or metal fasteners' outlet ( they don't call them " nut and bolt shops " ) along with a packet of choccy Hobnobs ( it helps ) and ask for a longer one.
Are you sure? The centre bolt on a side-pull brake of any quality tends to be a specialised item with threads both ends and a built-in boss for the spring etc. You may well find that a Weinmann or other similar side-pull stirrup will have a compatible bolt. It doesn't have to be a 500, any depth will do.
Today is the yesterday of tomorrow.