While this is an old thread, the law listed there is out of date. Wisconsin has a clear law on "Motor Bicycles" but it is hard to navigate because it is jumped across a few statutes I did some research in this and will quote the current statutes and post links to where they can be found.
Definition of a motor Bicycle: Wi Statute . 340.01 (30) “Motor bicycle" means a bicycle to which a power unit that is not an integral part of the vehicle has been added to permit the vehicle to travel at a speed of not more than 30 miles per hour with a 150-pound rider on a dry, level, hard surface with no wind and having a seat for the operator. “Motor bicycle” does not include an electric bicycle.
docs.legis.wisconsin.gov
Motor bicycles are exempt by the state from a registration requirement with a small hitch
Wi Statute 341.05 (23) When vehicles exempt from registration. A vehicle, even though operated upon a highway of this state, is exempt from registration if any of the following applies
(23) The vehicle is a motor bicycle, electric bicycle, or bicycle, except as provided in s. 349.18
docs.legis.wisconsin.gov
s. 349.18 is the one that could bite you 349.18 Additional traffic-control authority of counties and municipalities. Relevant sections
349.18
(1) Any city, village or town, by ordinance, may:
(a) Designate the number of persons that may ride on a motor bicycle at any one time and the highways upon which a motor bicycle or moped may or may not be operated.
(2) Any city, town or village may by ordinance enacted pursuant to s. 349.06 regulate the operation of bicycles and motor bicycles and may by ordinance require registration of any bicycle or motor bicycle owned by a resident of the city, town or village, including the payment of a registration fee.
(3) Any county, by ordinance, may require the registration of any bicycle or motor bicycle owned by a resident of the county if the bicycle or motor bicycle is not subject to registration under sub.
(2). Such ordinance does not apply to any bicycle or motor bicycle subject to registration under sub.
(2), even if the effective date of the ordinance under sub.
(2) is later than the effective date of the county ordinance. A county may charge a fee for the registration.
docs.legis.wisconsin.gov
And lastly, the above-mentioned Wi. s. 349.06in particular s. 349.105 Authority to prohibit certain traffic on expressways and freeways. The authority in charge of maintenance of an expressway or freeway may, by order, ordinance or resolution, prohibit the use of such expressway or freeway by pedestrians, persons riding bicycles or other nonmotorized traffic or by persons operating mopeds or motor bicycles. The state or local authority adopting any such prohibitory regulation shall erect and maintain official signs giving notice thereof on the expressway or freeway to which such prohibition applies.
docs.legis.wisconsin.gov
so ultimately, the best advice for Wisconsinites is to check with their city and county.
Disclaimer: I am by no means a Lawyer, just someone who was willing to Chase these laws around like a rabid dog.