I actually put together 2 of these. The first one (black frame in the photos), got stollen shortly after I built it, so I decided to build a second one, this time starting with a better bicycle (aluminium frame, better seat, better gears and disc brakes).
The engine kit is widely available on Amazon for about $150, makes about 5hp and can propel you as fast as 35mph.
The second build poised some challenges, since the hubs were different because of the disc brakes, so a new sprocket and 3D printed hub centering rings were needed. Overall it turned out great and better than the first.
I will keep posting updates here as I go. I recently designed and printed a housing for my headlight switch.
Here is a video of the second motorized bicycle in action:
And some pix: