For reference, I used all the pictures I took in the early '90s, I have old park maps and brochures, I used Google Earth extensively, and I used greatamericaparks.com and the book about Six Flags Great America that the site owner put out. I spent a crazy amount of time on research.
I incorporated the large cartoonish park map into the build for the map/information signs spread throughout the park.
Glad you like it! Here's my workshop link: https://steamcommunity.com/id/olsor54/myworkshopfiles/?appid=493340
I've been working on the park for 3 1/2 years now, and it's gone through a bunch of revisions as I've learned better ways to do things. I just spent two hours last night reworking my Cajun Cliffhanger building (even though there's no actual ride).
By the way, Steven Wilson's book on Great America is a nostalgia blast.