Afaik, those are not really knockoff as in a Gameboy replica. They're still using the proprietary Apple mainboard and all for the most part. What they did to double the storage was to desolder and solder back in a different storage with different capacity with the image from the original storage cloned onto, a process which kinda costly amd very risky as the components aren't designed to be plug and play like desktop components
Ref: Youtube channel "Strange Parts"
TomMarius
There are stores selling knock-off easy to manufacture parts like body, some basic electronic components etc, and yes, as I said, the logic board itself needs to be original.
Ref: Youtube channel "Strange Parts"