I see that many newer Sunfish have a horn cleat on the starboard side of the mast. My older rig does not have one. Is that something I need to add?? If so, what is the best way for me to attach it to the mast? Aluminum Pop Rivets? Screws?
I used screws, and like JohnCT, put it as high up off the deck as possible. I used the 3M 4200 sealant behind the cleat, and fastened it on with #10 stainless steel sheet metal screws – I had to pre-drill the mast with small pilot holes.
this picture is a good diagram of where to put it, as well as other rigging information.
If you put the cleat too high, you won't be able to reach it in waist deep water. You might consider that too. Lastly I think two properly drilled holes with screws will not be the weak area in a Sunfish mast.