First *nix that comes to mind is NetBSD which had 68k ports a very long time ago. Even when I was still active using my Amiga back in the last 90's this 68k port existed so should have alot of good documentation around and time to mature.
Even though its not linux might be something good to look at. I prefer BSD style since Linux can get a bit bloated at times.
http://www.netbsd.org/ports/amiga/
