Did you ever get the measurement of that sheave? I got to replace both my upper and lower sheaves on that mast. They were plastic and were cracked. Hope to replace with aluminum.
I would remove them George and take them somewhere to match them up. They need to be able to handle 5/16" halyard. You're going to need to drill out the rivets on the lower one. The mast head sheave is held in there with a pressed pin I think. When I replaced my mast head sheave I used a stainless steel bolt and lock nut in place of the pin as Rudy suggested. The bolt should be smooth for the pulley and have just enough threads to attach it to the mast head.
I bought a block with the sheave in it off Rudy for my Jib halyard and had to Mickey Mouse it to fit my mast which involved cutting out a slot in my mast with my Dremel Tool to fit the block into it. The two flanges on the block that fit on the mast, needed to be bent to conform to the shape of the mast. There are holes on the flanges for screws or rivets. Then I had to bore four holes in the mast and tap them for threads for the stainless steel machine screws to hold the block in place.
I would remove that cast Aluminum block and try to find a metal sheave that will fit it. Then you can rivet it back in place later. The Jib block on my mast looks a little "hacky" but I only use my Jib Halyard for raising my mast as the CDI has it's own halyard. Have fun with that George! I did.
Joe