>>1517316people used to rig up their own E bikes, Ive seen them
mostly the system involves relays to switch between resistor banks to control the performance of the motor
and of course lead acid battery's powering brushed motors
these days you could just buy lithium battery's in bulk and hook them up to almost any variable frequency drive that can handle the current
this along with brush less motors has made things much easier
the hard part is now transmitting the drive without damaging the frame or drive line
that and managing to have a decent weight distribution
or you could just strap a chain saw to your bike, simple as
