Old Powerbook 12, newest MLB

MLB = logic board / motherboard? My impression is that the very first Powerbook 12" is different from all the ones after it. In particular, the ports are not the same -- the original 12" PBG4/867 had a mini-VGA port, while all the others have mini-DVI (which is bigger than Mini-VGA, I believe). So I suspect that, without some hacking of the case, you may not be able to get the board into that model. I think, unfortunately for you, all other 12" PBs are interchangeable, except for that one. See this:

http://docs.info.apple.com/article.html?artnum=88248#12

(The one they call PBG4 12" / DVI is the second revision model that clocked at 1GHz.)

I doubt that swap will work, but you can check the guides at disassembling the PB at http://www.ifixit.com/Guide/Mac/PowerBook-G4-Al-12-Inch/53/ and see if its doable.