I am considering to make the same move. Provided that someon already has a Mac Mini G4, besides MorphOS what else is needed to make it work as an Amiga PPC?
MorphOS can run Amiga software out of the box. There are also PowerUp, WarpUp and Warp3D compatibility layers so you can run old Amiga PPC software there.
But it cant run anything on Mac mini that would hit the hardware directly and you might experience compatibility problems with some old software.