MorphOS and OS4 aren't quite the same since they're not (AFAIK) API compatible with OS3.x. A PPC rewrite of 3.x would be both API and binary compatible (binary compatibility being provided by the 68k CPU).
There is no binary compatibility without API compatibility. They are not 100% compatible because they dont support everything (i.e. BCPL programs).
Is there any reason why OS3.9 functions couldn't be rewritten as PPC versions?
One could also write a 68k emulator for PPC (to run OS3 on PPC under emulation) and systemically replace 68k components.
