I've not raced Geneseo but have done a few WFO races. No need for a spark arrestor, your KX will be fine. It will probably be tight in some places and wide enough to pass in others. Muddy if wet, same as everywhere in Illinois. The big-bike races usually start at 12:00 or 12:30, bring your AMA and D-17 card if you have one; if not you can buy them at signup. Should be a good time - bring a drink system, will probably be hot and the races are usually 1 hr 45 minutes plus one lap (probably 2 hours in total).