If you do head out to Prairie City, ask for Bill McCallister. He and his wife Paula are running the show out there these days, and he will answer any and all of your questions. He will get you going in the right direction, and he has a couple of rental karts, so you might be able to check out a kart before buying one.
The kart track is located all the way at the back of the park, near the water tower, ‘igloo’ and park offices. There’s a sign showing the way at the ranger check in station.