Wis. Stat. 344.01 b defines motor vehicle as "a self-propelled vehicle......."
Wis. Stat. 340.01 (30) defines motor bicycle as
(a) bicycle to which power unit not an integral part of the vehicle has been added to permit vehicle to travel at speed of not more than 30 mph....
(b) 2 wheeled vehicle that has fully operative pedals for propulsion by human power and an electric motor of less than 750 watts and capable when powered solely by the motor of a max speed of less than 20 mph....."
It appears that liability insurance would depend on whether or not the motor bicycle was self propelled like a moped. For example, if the bicycle is just electric assist and cannot do anything without pedaling, then it would not be considered a motor vehicle.