Re: legality and choosing engine displacement, torn?
My guidelines for building are: predominant riding terrain, local legal requirements, rider and budget. Most places define a motorized bicycle as 50 cc or less. Here in Tennessee we also have a motor driven cycle category. There are lighting, braking, horn, registration, insurance and motorcycle license requirements. You're allowed 125 cc and 5 HP. Many scooters, pocket bikes and mopeds fall into this category but you can build bicycle in it as well.