I recently replaced the broken cheek block on my mast. It was easy to do. Drilled out the old rivets with a 3/16" bit. Installed the new block with new rivets and caps.
Important: I would not attempt doing this with the typical rivet gun you may have in your garage. 3/16" stainless steel rivets are tough. I was able to borrow the rivet tool in the attached link from a mechanic friend. It is a $250 tool.
https://www.afsmarson.net/Products/Tooling/Hand_Rivet_Tooling/39031.php?l=enIf borrowing is not an option for you and you don't want to spend $250 there is a "Heavy Duty" rivet tool at Harbor Freight for $19...however, I have no experience with it at all. It is probably perfect for intermittent use.
https://www.harborfreight.com/heavy-duty-17-1-2-half-inch-hand-riveter-with-collection-bottle-66422.html