Police here in my town pulled over a guy on a mower and arrested him for DUI...
They can and will stop you and ticket you for having an unlicensed and/or unregistered vehicle...there is ALWAYS something they can stop you for...
but above all, I would not do it because it just would NOT BE SAFE to do...brakes, wheels, tires, bearings, frame...NOTHING on that mower is designed or engineered for the stresses that those speeds would put on the frame and drivetrain. You would end up hurting yourself...or worse yet, some innocent family driving down the road!