I'm also in Il., in Peoria. First off, almost everybody goes ahead and uses the 66cc engines if they want a 2 cycle. Just make sure there is no label stating size on the engine and gear it low for slower speed, also better for hills.
I'm like you though, I want to stay small, although I will probably go closer to 32.5cc.
Check out this page from Staton-Inc., a very reliable company.
This page shows all their engines, some are sold singly, others are available only with a kit. They have lots of pages showing it all, so be prepared to browse awhile. They definately have a Honda 25.3cc and a Robin-Subaru 24.5cc, both small enough for your requirements. Quiet and reliable 4 strokes.
For more information on the climate of riding in Illinois, go down to the Laws and Legislation section in the Tavern, look for the thread "Illinois". You may want to skip to within a few pages from the end, it's long and was started before the new law if Jan 1, 2010. It gets pretty involved to say the least. I'm about to add to it now.
Whereabouts in our great state are you?