Hi Pokie - here are a few within 100 miles:
Warren, IL - 25 miles -
www.racethestorm.com - Stadium style, not sure if they have open riding
Mt. Carroll, IL - 35 miles -
www.mcmotopark.com - Probably your best option - Outdoor MX style, lots of elevation changes, safe, fun, called a "mini Red Bud" by the promoter, has open riding
Lena, IL - 35 miles - Lena was listed to have MX races either in 2003 or 2004, but I don't think it worked out - wardy may have more info
Morrison, IL - 55 miles - Morrison had a MX race a few years back and the track was said to be an awesome outdoor style MX with elevation changes and like a mini Red Bud as well, I think the racing thing didn't work out because of noise, but the track might still be open for practice, Brush Poppers runs enduros and hare scrambles in Morrison and they might own the track as well - 815-626-4756.
Pecatonica, IL - 65 miles -
www.statelinemx.com - Stadium style, not sure if they have open riding
Fenton, IL - 65 miles - Fenton was listed as a MX track a while back as well.... not sure on the status
Cedar Rapids, IL - 80 miles -
www.cvtr.org/framemain.htm - Hawkeye Downs stadium style, not sure if they have open riding
Byron, IL - 85 miles -
www.motobyron.com - Pretty sure you've been there ;)
Walnut, IL - 90 miles -
www.sunsetridgemx.com - Outdoor style MX track with elevation and natural terrain, very safe, fun, somewhat sandy/loamy, wide and multi-lined, has open riding
Coal Valley, IL - 90 miles -
www.tricitym-c.com - Outdoor sytle MX track with elevation and natural terrain and some man-made doubles, whoops, tables, somewhat hardpacked, fun, has yearly club memberships with open riding