I'd reccomend the apple service guides they are pretty descriptive and show you all the steps and pics you need to do to pull your book apart. The trick is to take your time, make sure your earthed before you start. watch out for the little rectangluar magnet under the bottom right hand conrner of the keyboard, below the top case, it activates the reed switch in the lid which sends your book to sleep, I forgot it the first time and my book didn't sleep on reassemble...

also check the mod guides for ibooks on this site and any others you can find.

I'm not sure how easy it would be to swap the motherboard, I've never needed to got that far before (though if the usb mod goes wrong i might!)

but from the looks of the manuall the thermal pads between the chips and proccesors may need replacing after, depending on condition, don't know how thick they are so you might be able to use thermal paste instead.

you should find the manual for you ibook here:

Hope thats of some help to you.
I break stuff.