​Fair enough... the Mt Xinu version is pretty much the CMU version unadorned.   Which mean that it is a 4.3BSD kernel, with the BSD based MMU code ripped out and replaced with the CMU code, and the Mach interfaces (ney RIG - Mach's and Accent's predecessor) messaging system spliced into it; then the whole mess was built back up using a 4.3BSD user space (and on top of the i386, an Intel developed boot system - which is a different story I'll not repeat at this time - but thankfully was common to all the UNIX systems of the day because Intel developed and make it available to community at large).

The other option which I would suggest to look at is the OSF/1 mk for the i386 (monolithic) about version 3x which as I said forked off the Alpha line and a couple of other systems.   The i386 version of OSF/1 supports the same chips (i386/i486/Pentium) at the CMU version, it also comes with more HW device support (disk, tape, network, display et al),  than the CMU/Mt Xinu version -- including most importantly SCSI support by default, which is why is might be a little easier to work on today's HW and VMs.   When I last used it, it lacked USB support; but that was being worked on around the time I started doing other things so even that might even be available today.

FWIW: OSF/1 also started with 4.3BSD userspace, but it had a lot of work done to it to updating it - adding the Sys V commands that BSD lacked those days and adding Sys V options to many commands.   i.e. its user space is a tad more "complete" / "wider" than pure 4.3BSD and again makes it a little easier to complete. 

Note that the user space commands from the mk would become the basis for Tru64, HP/UX and later versions of AIX.   And also the OSF/1 version will have better Graphics, Motif and a much better GUI options all around that Mt Xinu, which alone may be it easier to work.

As I also said elsewhere, the uK or Research Institute (RI) version is a tad more fun to play with.   It's a real kernel architecture moving things like file systems et al in user space.  But you can do do things like start up multiple system interfaces.   LCC had their DOS/Win95 interface was actually developed running instead of as a VM like it did for the basic mk code, but in as "second server" but I do not think they ever sold it.   The other thing the RI never did, was the uk still has the pager and all the networking code in the kernel, so the uk, is hardly 'micro' in size.

There is a OSF Version 4 and maybe even version 5 (I've forgotten, if some one remembers - please correct me).  The OSF RI folks were trying to rewrite it a bit in C++ as I recall, again this part of the UI vs OSF wars of the day and Chorus has rewritten there version from Pascal to C++, and IIRC the RI was trying to counter that.  I don't remember if that version of the uk ever saw the light of day.

Anyway, no matter which is the 3 code streams you pick, Mt Xinu, OSF/1 mk or uk one hardest problems for today will be that the compiler is of course extremely old by today's standards, and you are probably going to run it some walls in that area faster than you might think.   That said, if you are willing to deal with the compiler as it comes, non of them should be very high, or hard to get clear, but some are likely to take some work.

Have fun and good luck and let us know if you can get any of these running.

Clem