Handheld retro PC design concept: Commodore HX-64
artstation.com
artstation.com
There are however some other projects out there doing this. See http://mega65.org/
They have a handheld, doesn't look as nice as these drawings though.
The Mega65 system, replica of the commodore 65 is awesome though. It is compatible with c64 c65 and their new system Mega65.