Tor-Eirik Bakke Lunde a.k.a TEBL created several computer systems with detailed documentation.
The RC-ONE is a KIM-1 clone, inspired by the design by Ruud Baltissen, which can be built in a compact to much expanded format.
TEBL separated functionality into separate boards so that each version would cost only around 5$ per piece instead. It means it’s no longer a single board computer, but if it means more people can build it as a fun project then that’s a plus in my book. The main setup does not include the additional 15 I/O lines due to space requirements, but you can add those back to the system by including the 65K Expansion (in addition to the backplane module) if you want to. If you just want to play around with the system and try your hand at machine language coding, you can get started with the CPU board, UI board and a keypad module.
The whole design is documented on TEBLs github page, from circuit diagrams to PCB design.