The earliest Unix running on the 11/20 requires not only the custom MMU, but also a fixed-head disk
Right, a 4Mbyte RF11/RS11 which Ken refers to as /dev/drum in this code. That lived for a pretty long time - at least thru the Sixth edition, IIRC.
(the genesis of the tiny root partition?),
I thought the original system just swapped to it, no mounted FS on it, but unless someone knows for sure, it means staring at Ken's assembler listings for V1 that Warner mentioned earlier.
so I agree with Noel that MINI-UNIX and LSX, while not "historically correct" for your hardware are a good fit, and that
building a working system on simulated hardware would likely be the easiest route.
Phil