While the motor kit did not come with any installation instructions, It seems there are a bunch of videos on line that explain the process fairly well. I don't think that all the 50cc motor kits are made by the same company, but I would venture to guess that there's a lot of over-the-shoulder-looking going on in China.
When they can't spy on us and steal our technology, they spy on each other, hence these kits look a awful lot alike. What they don't mention in their ads is that the kits seem to be oriented towards mounting on a one-speed heavy duty bike. What we used to call a paper boy bike. Assembling one to a mountain bike could present some new challenges, but at this point it looks like the big items are accounted for.
Going over the assembly videos, I've accounted for about half the fasteners that came in the plastic bag. The rest must be spares to be used when the first set vibrates loose and falls off. Note in the picture 2 posts down that there are 2 large sprockets, one plain and one with a pedal crank attached. I think the pedal crank with sprocket is intended to go on the right side of a one-speed bike to deliver a lower starting ratio to compensate for the extra weight. The larger one gets attached to the rear hub on the left and is driven by the motor. The hole is sized to fit a coaster brake hub. I'm guessing I'll have to make an adapter to go between the derailleur hub on the mountain bike and the drive sprocket to get it to center up.
The pedal crank with the sprocket goes on the right and has 5 holes in it that don't quite match a derailleur set of chain rings. Some modification may be in order here.