Need tall? a threadless conversion will work
the headset will slip over the unthreaded part of the fork.

spacers and BBB BHP 20 wiggle.com | BBB BHP-20/21 Stem Extender | Oversize Stems will give you the top of the steerer tube adjustment of that headset

Motorizing a bicycle (on the cheap) usually results in having poor braking capacity..

Why not just buy a MoPed and pay the road tax & licence fees?