Missouri's moped laws (motorized bicycle laws, they call them) require 49cc or less, no more than 3 hp, top speed no more than 30 mph, automatic transmission, and I think a headlight visible for X number of feet and taillight visible for X number of feet. You have to have an operator's license but you don't need to license it or insure it.
I've been using my hardtail Z50, with its, ah, hmm, "automatic transmission" as a moped for years, on and off, since it meets or exceeds the other requirements. I've not been stopped on it yet. But I've been in rural areas with low traffic since college. In college I commuted all over town on it but the cops there were used to seeing college kids on mopeds and scooters so I guess I blended in.
There was one other kid I saw around back then with a totally cherry '69 or '70 Z, a red and white one, who rode it to school occasionally. Never talked to him about it though. Wish I had.
I want to get one of mine actually LICENSED, with an engine big enough to go more than 30 and the ability to not "act like a moped" whenever a cop comes along. I just need to decide which bike now...