To start, Forgery in the State of Wisconsin is a class H felony punishable by up to 6 years in prison. So, that is the potential maximum penalty for having a forged state ID.
But not withstanding hypothetical penalties, you need to ask yourself why you want to go back so badly. There must be plenty of other gyms that you can go to. Frankly, this seems like a lot of work and risk just go to the gym.
If the faked ID actually contains another person's biographical information, it would also be punishable as identity theft under federal and state law. Under Wisconsin law, identity theft is a Class H Felony, currently punishable by a fine not to exceed $10,000, imprisonment not to exceed 6 years, or both. Granted, those are maximum sentences, but not something to mess around with.
This response is intended for informational purposes only and does not create an attorney-client relationship or constitute legal advice.