First of all www.Dieseljo.com
I'm starting my second year on the ice. I suck, and go pretty slow.
For us on the south end of the city, Moto On Ice is a series that takes place in Cedar Lake Indiana. Typical day includes oval racing in the AM, followed by TT course in the afternoon. You can race oval, TT, or both.
Not a ton of classes. They run well seperated in the morning, pretty much all together in the afternoon.
Up north there are several orgs, CWIRA and maybe SWIRA are in central and southern WI respectively, I have no experience with either.
Buy a set of tires. I know, they're expensive, but I would hate to see people not like it because you're fucking with equipment. I have a my ice tires on a spare set of rims, so I don't have to fuck with mounting them every season.
Buy a decent set of wraps. It's a pain in the ass with good ones, and your tires will be destroyed without them.
Buy/build decent fenders. You'll need them to race, and everybody likes to go home with all the flesh they came with.
Bike doesn't matter. You'll see everything from one off rotax powered flat trackers, to xr250's with sidecars. Any mx or offroad bike will suffice. 2t's are a bit easier to start in the cold.