I am dealing with this same issue and unfortunately, my original fastener is long gone. They do not sell replacements that I can find. After some quick measurements, a 5/16" x 7/16" long shoulder bolt will do just fine. Add a 1/4 -20 nylon lock nut on the other end. Should be plenty of room. Eliminates the push nut. I had to order the bolt from Mcmaster Carr and about the same price as a 5/16" pin with a push nut.
If you have a good hardware store nearby, you might be able to find the pin. Push nuts are easy enough, it is the pin that could be difficult.
I had a plan B. Then remembered I had another assembly in my part stash.
just shorten the thread and it will do the job. Can even get a socket through the access hole in the brake assembly. ACE Hardware again.
The only issue with plan B is that threads don't make a good bearing surface. Although in real life, how often will this P. brake be used in a year. That is why they used a solid pin from the factory or could use a shoulder bolt.