Hi iamwatching,
I love worldtimers too, but have given up on finding any that looks HALF as decent as the gen ones. Read my findings/reply to another member who was also interested in the same enamel piece you've tagged:
https://forum.replica-watch.info/vb/showthread.php/176107-World-time-5131 (or you can do a search on the thread "world time 5131").
I've recently come across a chinese instagram account who claims to have the piece (he posted a photo of the rep, same bad quality as the pic i've posted on the other thread). I'd suggest to stay far away.
If you really love worldtimers like i do, I'd
recommend forking out the money for a gen piece, and it's possible to get one from Orient (
under 1 grand; they have several designs), Longines or Ball (under 2 grand).
As to the question to if they're too hard to rep, i think it's not too hard, but it simply doesn't make economical sense to the rep makers to remap and come up with a new assembly to even build module onto existing movements, what more to replicate a complete worldtimer movement. Not many people (layman) are even aware of the fact that there's such a watch function (at least my non-watch-loving friends were amused when I showed them my Orient's worldtimer function).
Good luck in finding THE ONE!
metalaphid