Corrected entry: In the beginning they show an Illinois license plate with OPA! on the plate. There are no punctuation marks on Illinois license plates.
Correction: They could have gotten one that says "OPAI" and just used a little paint to touch up the I into an exclamation point. Probably illegal, but doable.
After reviewing the scene, there's no possible way it was done with just some paint (nor is it possible in real life to do). Illinois license plates don't use any letters or numbers that resemble a lowercase "l."
Correction: The front plate seen isn't state issued. It's a novelty plate he put on the front. While it would be illegal for him to drive without a state-issued front plate, there isn't anything otherwise preventing him from having a novelty plate on and cops probably wouldn't harass him about it. I drove my last car for 10 years without a front plate in a state that required it and never got a ticket for it, even when I was pulled over for speeding and the cop told me I had to have one.
Bishop73