Motorcycles have two or three wheels, a seat for the rider and at least a ____ horse-powered engine.

depends on state laws, definitions. 'Motor Driven Cycle/motor bicycle' not allowed on interstate is less than 150cc and some places have a 15hp minimum- was Ohio minimum few years back. Motor assisted bicycle or moped in some places like New Jersey or UK has a 2 hp, 50cc upper limit with a speed restriction of 20 mph or 30 or 31 mph- if more than that it becomes a 'motorcycle'