I've got good news and bad news.
The good is that you can buy that bolt cheap at any hardware store in a couple grades of hardness, 5/8ths coarse thread (3/4 for late model trunnions). Probably have to buy one over length & shorten it for a match. Drill & tap for a zerk or fill your trunnion with grease and force the grease in as you turn the new bolt in.
The bad is that the threads in your trunnion are partially buggered too and a new bolt will tighten it up some but won't restore the joint to 100%.
I may have a used set in a parts box but at best the threads will show some wear on one side.