As a maybe 13 or 14 year old, the manual for the Osbourne2 Z80 CPM machine dad bought contained enough information for me to work out how to connect a broken Atari video game joystick to the parallel port, and how to read it by peeking memory from Basic...

Circuit diagrams, bank switched memory details, pretty sure someone dedicated could have built themselves their own machine with access to the manual and the right rom dump and system floppy disks...