Interesting stuff guys. I am of like mind when it comes to physically altering any of my older/classic/historic tools or machinery. I just can't bring myself to cut, drill, or weld anything to it if I can find a way to make it work the way it is.
The synthetic belts are worth considering. A person could look into some of the flat serpentine timing belts in use today but getting them onto the wheels could be a little problematic (read that as PITA). The permalinks can come apart and be re-hooked if they are what I am thinking of and would probably make an excellent backup. It's good to know that even if parts are not out there, they can be made/gotten. When it comes to things that are "universal" though, tool posts, etc. would it be best to get items designated for 12" lathes or 10" ones? I would assume you'd go with the 12" depending on exactly what you are looking at.
Okay, here is another stray thought of someone who isn't that familiar with machining and lathe work: I do wonder though, with belts that provide a better grip than leather or some other drive configuration, whether that would be best or safest. What I mean is that the system was designed with the leather belt in mind. If something were to bind up the leather belt would probably allow enough slippage to prevent even more serious damage. If a "better" grip were more desirable a person could take it to the extreme and just replace the pulley system with a chain and gears, no slippage at all. So there is the question, is using the lathe as originally designed most likely to produce the best results.
-Ron