From my experiments with this a few months ago, I don't recall hearing anything in the 1820m's built-in effects that sounded anywhere near as bad as a spring reverb!
What I want to do is set up what is called part 15 AM station (with about only 2 mile coverage area) and I am trying to get a similar reverb effect that a station I remember hearing (circa 1977 also on AM) had. I have been told that that station used a Fairchild 3 spring reverb in it's airchain. I guess any non reflective (bouncy type or slap back) type of reverb effect would be comperable. I guess a spring reverb sounded bad, but I don't think I've really heard a good enough recording of one to know. I have a recording of the station I'm trying to simulate, but due to the fact that the transistor radio and tape recorder used to make the recording were not good quality so it's difficult to say.