Modem emulators will not do LocalTalk. LocalTalk is not RS232 and is not point-to-point, it's point-to-multipoint, which has historically been a right pain in the backside to do well on the Internet (until a year ago this was what I did professionally). It's not just a point-to-multipoint thing, though, LocalTalk on the wire is decidedly different from RS232.
Some modem emulators, which just backhaul the whole serial connection (so are suitable for, say, BBSes) run AppleTalk over PPP or ARA or similar, but you'll need the other end of that somewhere else. Farallon Liaison will also do AppleTalk over dialup and that is on the Garden (I know because I uploaded it). You may be able to get away with putting the far end of the dialup connection on an emulator somewhere (I have vague memories that
@Realitystorm did this at one point, but I may be misremembering), and that would give you approximately what you need. That was what these dialup bits were designed for: to have a distributed AppleTalk network over standard long-distance RS232-ish links.
A more direct option would be that bbraun did a kind of "AppleTalk VPN", but it might be tricky to get working - I've never tried.
An option that will give you more speed and more flexibility but more complexity is to use IP tunneling. Apple Internet Router will do this for you, which is what I believe the GlobalTalk people are using (though I don't participate in GlobalTalk myself). Other router manufacturers had similar stuff; I have briefly run a transatlantic AppleTalk network using Gatorboxes, for example. I'll note, though, that these don't follow modern best practices in a number of areas (because they're not modern, so that's hardly a criticism) so expose them to the Internet with care, and don't expect too much of them.
AirTalk will only help you in one of two circumstances: you create a big virtual broadcast domain with something like ZeroTier, or you invent a time machine, go back in time and make the multicast internet not fail. Both of these present practical issues.