Aftermarket CPUs normally work without issue. I know a Sewde who has a 1.2GHz sonnet in his G4 and uses OS9 just fine.(it is mystic too iirc.) I have also never had an issue running 9 on my sawtooth or digital audio with my 1GHz card installed...but my current gfx card never had support and my SATA card cannot boot 9 at all.

I used a 9.2.2 "G4" disc though, but I have also used a retail 9.2.1 disc too unless I am mistaken.