I think Linux is the best choice to enable latter day
retrocomputing.
Check out the ELKS project to put Linux on machines with very limited
resources:
On the other hand, CP/M-68K is available from
http://cdl.uta.edu/cpm/.
A lot of it is written in C; with some work, it can be modernized and
updated. What could be more retro than building the ability to port
CP/M to anything with a C compiler?
Roger Ivie
ivie(a)cc.usu.edu