Sorry I can't help with Fond du Lac info. Driven through there several times, but haven't seen anything to ride. One of my favorites from the mid-late 70's was the motocross races at Little Switzerland ski hill. I have no idea if they still do that or not.
But my sister has a cabin near Presque Isle, WI and I am looking for trail in the UP. I understand there is a lot of National Forest areas that are open to riding. Do you know of trail close to Presque Isle?
It may be as long a drive as getting back to the UP, but I also hope to do some riding around Black River Falls. I've heard there are some pretty good trails over there.