The funny thing is, the "old school system" doesn't appear to be the first! As I started disassembling the outhaul I discovered a hole in the top of the boom and can see a internal sheave!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.
BYW, contacted the sheave place you linked to. He says he can replace them at $30 a sheave, and that they will handle wire. However, I'd much rather go with Dyneema. Actually, I'd much rather replace the whole system if practicable. Trying to convince the end cap to come off today....Replacing the wire with dyneema is a pretty simple conversion, and absolutely worth doing.
I have a very similar system on my boom, if you can get the wheel good for you, I find this outhaul works great...I also have a small winch up forwardThe nylon wheel in the first picture has worn out. Anyone know where I could find a replacement? I wouldn't be opposed to replacing the outhaul altogether.