With a 16 mile range, and intention to pedal and not throttle, you could just put a friction drive on your bmx. Easy to add, you might wear tires a bit more often depending on the drive. If you plan to ride in heavy rain or snow, probably not gonna work for you. Endless Sphere can help you make your own (Kepler might still be selling his kit) very inexpensively. There are some commercial ones available as well (OneMotor, Alizeti, Add-e, Go-e, Rubbee). Folks have been using them on folding bikes (small like bmx) for a long time, Brompton/Hidden Power for example.