Bought the downhill MTB used, then pieced together the rest of the electrical stuff from various suppliers online. Most of the parts are from the US, rear wheel was laced in Sydney. Drilled extra holes in the motor for better cooling. I had torque arms/plates laser cut in Canada from strong steel to prevent the motor from tearing apart the bike. The computer is called a cycle analyst and manages the battery pack, plus provides a nice running dashboard and various statistics.
I'm better with electronics and software than metal fab/mechanical engineering type stuff. Sorting out how to re-enforce the rear dropouts (where the axel goes through) took me the longest by far. I don't have any welding gear, so ended up sandwiching the steel and aluminum parts with a very strong 3M metal epoxy, then bolting them for added strength and alignment.
It took a few months, but that was only working on it in my spare time.