That is one seriously old school system. For what it's worth I would go to an internal cascade system with a clew strap. But there are a number of places to get those sheaves. Zephyrwerks - The Sheave Factory
custom makes them for about $30 a pop, or you can try to size match with either harken, Schaefer, or Lewmar. You may have a problem finding wire sheaves these days however since wire rope is becoming increasingly rare.