I think you might be underestimating the requirements for a UNIX like OS. An 8Mhz 68000 struggles, so how a 6502 is supposed to cope I'm not sure.Might want to investigate Minix. It's designed for teaching OS design, so even if a porting effort failed, it would be an educational fail. It is to some extent unix-like.
What you have to do is stretch the definition of UNIX as much as your brain will allow in order to call the small bit of code you shoe horn into the box as something vaguely reminiscent of an idea they once had back at Bell that may have been somehow related to an operating system. If it doesn't have fork() then it ain't UNIX.