You could probably come to an easy agreement with Frank Mariak:
You get hold of the entire 68k Amiga sourcecode and binaries and the right to do with it whatever you want. And you could offer him the same deal in regards of the MorphOS development and binaries stuff.
So it would be mutually exclusive and beneficial.
You keep ownership of 68k Amiga stuff exclusively.
Frank Mariak keeps exclusive ownership of the MorphOS stuff.