I would give Rudy Nickerson a call on the phone.
http://www.drmarine.com/ I replaced the sheave in my Z-Spar mast head a couple of years ago. Fortunately I only live about 8 miles from Rudy's store and was able to remove the mast head and the sheave and bring it down to him to me sized up.
The sheave pin on my mast head was pressed in place at the factory and I wound up using a threaded stainless steel axle bolt with a lock nut. Rudy said that this is a permissible procedure when replacing the pin. All I had to do was drill the hole diameter a little larger in the mast head to accommodate the bolt. Rudy found a sheave of the right size with a stainless steel bushing to fit inside the sheave.
If you can't use the same pin you may be able to use a bolt and lock nut too. It's worked great through the years.
