With something like Shapeshifter, older (non-PPC) versions of MacOS should run on an Amiga. I also think that there are 68k ports of various BSDs.
MorphOS won't run on the "Classic" Amiga. It's a PowerPC OS, like AmigaOS4 in many ways, and needs a PPC processor. Why you'd want to run another OS on a "Classic" Amiga is beyond be though :-)
