I haven't seen them for many years but the concept isn't new. My dad got me a 2 stroke 'kit' for Christmas when I was about 9 years old. I had to assemble every part, bearings, piston, rod, carburetor. That was many, many years ago. It was sort of like the 'Visible V8 kit but a real engine that ran, if you put it together right.
I'd imagine if you got with the right people that an engine 'Kit' wouldn't be that hard to market. I just don't know who those "right people" would be.
Tom