My Catalina 27 uses the same method of attachment. You want the bolt to rotate to allow the metal tang to swing when the boom moves side to side. If you pull the cap you'll see how it is fixed. I made the mistake of trying to tighten the bolt.... and it eventually released! Pretty hectic for a few minutes but using the vang saved the day.
When I pulled it off to repair, I drilled a hole in the end of the new bolt to accommodate a cotter pin, and used a nyloc nut for extra security. A double nyloc nut with a washer would work well, too. In addition.... I wrapped a nylon webbing strap (a sail tie, actually) externally around the boom and through the mainsheet's shackle. I saw this done on number of racing boats. Anyway it's held for at least 20 years now.. I'm happy... Just thinking about the subtle swinging the boom makes when docked or at anchor, besides when you sail, justifies the movement allowance. The reason your tang is twisted is because the bolt isn't allowing this free movement.
Finally,,, you could consider converting to some kind of bale through the boom end, as I did, for like 10 seconds, but then realized the nylon strap was the simple and best answer. It's easy to inspect and replace too.
Oh.... I forgot to mention this. If you are able to remove those stainless rivets without damaging the boom... which will be a challenge, for sure... use short self tapping machine screws with locktite for replacement. It is important to not damage the aluminum end casting... Good luck. Don't forget the STRAP