Oh, you are in for a long ride, I'm afraid! I live in IL too and I got a huge hassle when I bought my moped because it had never been titled...
The VIN should be enough for the State to be able to do a title search if that 'ped was once titled in IL. But it will probably take forever for them to do it. And then they would need a copy of the original title showing that ownership was transferred to you. Which means the person who is named on the title as the owner of the 'ped would need to sign it over to you and give you a Bill of Sale as well. If that owner can't be found, there is no way the State will put the title in your name. For example, if someone abandoned a car in your yard without a title, you would never be able to legally own that car. Also, you can't get plates without a title.
If I were you, I would take a pass on this. It would probably end up being more of a headache than it's worth. Tell the seller that you will buy it only if he can produce a title for it.
Your only other alternative is to go ahead and ride it without plates or title. But the fines you could incur would probably end up being more than what you'd pay for the 'ped.
Call the Secretary of State if you need more information.