What we have here in Michigan are ORV rules written for the dirt bike and quad crowd. Sometimes there is trouble relating that to full size, street legal vehicles.
As far as your specific questions, your factory exhaust system is all that is required. The muffler astc as a spark arrestor already.
Helmets are only required when there is no factory hard roof or roll bar protecting the occupants. Your Tj has a full roll bar fromt he factory. This is an approved set up for entry into the park. No helmet required.
For Silver Lake, you will need an ORV sticker and a state park Motor Vehicle permit which can be purchased anywhere a fishing or hunting license is sold. Also available at most state park offices.
You will also need an orange flag which touches a hanging board at the tech inspection station. I believe the height is 10 feet from the ground. There are stores and gas station near the park which sell these flags for around $20 or $25. They come with a nut and can be attached directly to your front bumper. Places in the area can help you with installation.
On busy weekends there is a voucher system in place, but that should not be an issue in April.
For any wheeling make sure you have solid tow points front and rear. Recovery equipment should at least consist of a recovery strap (looped ends, no metal hooks). A CB radio is helpful and can even add to the enjoyment with the banter that goes on.
For more information on the fuzzy Off Road rules in Michigan you can start here: http://www.michigan.gov/dnr
To see what you can do to support full size off roading activities in Michigan and get more questions answered, try here: www.glfwda.org