Thats much trickier than it sounds my friend. Apple parts are not often standard sizes or shapes (with the exception of drives), and nowhere is this more pronounced than in their laptops. As luck would have it, the iBook and PowerBook 12" logic boards are similar in size and shape but the PowerBook has an extra board to attach to it. Even with the similarities, the other ancillary parts have to connect in certain places and building the necessary extension cables would be a ludicrous job. Not to mention remodelling the case to accomadate the wrong board with good mounts and mylar or EMI shielding, plus heat sinks would probably need to be custom made too.
Also, the PB uses its metal case to dissipate heat. A PB board in an iBook case may be too hot.