I've done this twice and am planning to do it to a third moped very shortly. You get a Missouri bill of sale (I have 4 already signed if I can find them) and you take it down to the treasurer's office. They tell you that you can't register something without a title, then you explain to them that they are mistaken because the moped in question "came from Missouri" and they don't register mopeds over there. Then they look up stuff in a book and realize that you're right. Then they tell you that you need to get it inspected before they'll register it. Then you tell them either (a) I've done this 2 times before and I've never needed an inspection, (b) they don't inspect mopeds over there, or (c) here's the FAQ from the website that says I don't need an inspection. If you're lucky, they will usually realize that you're right about this too, at which point the "typing things into the computer" process commences. Then you sign 8-10 pieces of paper, pay them $30-40, and walk out with a plate. A few weeks later, they mail you a title.
So, long story short, if you send me a message with the VIN, I'll see what I can do about getting it titled. I think it'll be about $30-40 bucks for the registration/title, but I won't know the exact amount until I actually do it.