It’s next to impossible to give this advice because karting is continually getting more fragmented, with pockets of classes that are popular in one region that may not exist “next door”. So, if you just want to stay local, pick whatever is run locally. If you might travel or race beyond that level, the following kind of summarizes what might be available to choose from…
Shifter – is mostly “Stock” Honda, but the KZ (ICC/Euro) stuff is making a comeback in a few places. You’re correct that Mod Moto is all but dead, with the exception of some road racing, and the odd small club level stuff.
2-cycle racing – it’s a mess on a National level, but most regions will have 1 or 2 classes that are decent. In general, you’ll either look at “TAG” (Rotax Max, IAME, Vortex Rok each have their own 1-make deals, and in a few places they’re run together as “TAG” with a few other engine options); and some places still have an old-school KT-100 presence.
4-cycle racing – I’d say that the new(ish)Briggs LO206 might be the saviour of karting in North America. Any smart club and region should be running these things, and it is catching on more and more across the country. It might be the only class in all of karting that has a presence and is run to the same specs, from coast to coast. Aside from the LO206, 4-cycle is just as big a mess as 2-cycle.