While the ICA loves mountain biking too, the permitting and governance of mountain bike races falls on the USAC regional coordinator, not the Illinois Cycling Association.
A few good Illinois resources for mountain bike racing (and where to go mountain biking in general) are the Chicago Area Mountain Bikers, Peoria Area Mountain Bikers Association, and Rockford Area Mountain Bike Alliance. Our friends to the north also put on a fantastic series, the Wisconsin Off-Road Series.