Nice pic of that trike. I've been looking at getting an electric bike, as well as modifying my existing bicycle to be electric assisted. I'll probably do both. The legal limitations are that if you exceed the legal limitations (speed etc.) then you have to license it as a vehicle for the road. Apple has a lot of clout, and may be able to get these legal limitations modified. As you say, you can build your own with off the shelf parts. You can set it up to be just under the legal limits in order to ride it on the road; then add in an extra battery for extra speed and use it off-road.
I would look forward to seeing what the tech wizards at Apple would dream up. I really think they're working on a project like this.
I actually did retrofit a couple different bicycles with motors that I purchased from China through eBay. I put an 80cc engine on one and a 500 watt hub motor on the other. Neither was technically road legal.
The mountain bike I put the 80cc engine on was amazing. Because it was a 2 stroke engine, it had a “power band” which meant that if you were climbing a hill and let the speed get under 15mph it would start to bog down and lose power. It would easily do 35mph however and would maintain this speed over most small hills. Because it was so light it had a completely different feel than a normal off road motorcycle. Obviously, it was not street legal in any way.
The bicycle with the hub motor was silent when being used and would maintain 25mph on the flat, which made I technically not street legal as well, but I doubt whether the police would ever pay attention to it. It had limited torque but would still accelerate relatively quickly.
The type of brushless hub motor that I installed... worked by electronically creating a rotating magnetic field and when you increased the throttle setting the field would rotate faster and pull the permanent magnets within the hub around with it. The problem with this is that if you increase the speed of the rotating magnetic field to much faster than what you are actually travelling it basically loses its grip on the magnets and just slips past them and you lose torque. The problem is that this type of motor doesn’t do as well on hills as the wattage rating suggests that it would.
The bike with the electric motor didn’t quite meet my expectations. I installed gel cell batteries that gave it over 25 miles of range. It would go faster and further and cost me much less than electric bikes available already assembled. However the motor and batteries weighed more than the bike and we live in a very hilly area which the bike didn’t handle very well.
Both of them were proof of concept tinkering projects for me. I was able to sell each for quite a bit more than I had into them. I learned quite a bit that I didn’t realize before I put them together.