Thanks for the suggestions Zombie, but I was unable to find anything that was a good match. The most promising cheap carburetor I found was a chinese copy of the 42mm carb from a Polaris 500cc ATV. I'm sure I could have made that work, but it did not have provisions for the TPS, electric choke or bowl heater, so it would never be right. I am tired of spending time looking, so I broke down and spent the $372 for a new one last night. Out of the several online outlets for Kymco parts, Stadium Yamaha was the cheapest total price (part+shipping).
I am still thinking about that 42mm china carb. It would be an interesting experiment to see if that could be a cheap performance upgrade. I would have to test how the X500's ECM tolerates running without the electronic do-dads in the OEM carb. Or perhaps a TPS and electric choke could be adapted onto it. This sounds like a project for someday after I have the "right" carb as a backup!