That pin is probably still in low earth orbit from the last time someone worked on the brakes.
A bolt would probably work if you don't over-compress the spring. If I were you, I'd make a pin from a long shouldered bolt and either cut a groove in it for the horseshoe clip or drill a hole in it for a cotter pin and washer.
Use one of the other pins as a template to position the washer (where the groove would be) and the cotter pin hole to keep the washer in place.
There are probably people on this forum that have Huck brake parts left over from disc brake conversions. You might be able to get one.
As for the tool, I found a Channel lock pliers worked just fine to compress the spring and remove/install the horseshoe clip.
How did the previous owner secure the spring with the 5/16" bolt? Was the head of the bolt compressing the spring with a nut on the other side of the pivot arm?
