The stack of softwares is too deep nowadays to control everything excepting in the world of microcontrollers.
Check out http://www.cpcwiki.eu/forum/ and https://github.com/Octoate/cpc-sdcc-rom for some good starting points.
Amstrads can still be had on eBay for reasonable money, and one can gradually buy enough hardware to make a fun system. I just obtained a floppy drive interface that I plan to use to connect an SD-card floppy simulator to my 464 ... fun times :)
I've seen a modern OS for 6128 but I can't find the link.
There is this kind of spirit in the Forth language. I try to build something around it.
http://www.cpcwiki.eu/index.php/SymbOS
Screenshot here:
http://www.cpcwiki.eu/imgs/7/73/Symbos21cpc.gif
http://emucation.com/dl-rom-20511-group-6310.htm
... it's a Forth implementation for the CPC. I haven't tried it myself though.
The stack of softwares is too deep nowadays to control everything excepting in the world of microcontrollers.