You ought to be able to pull off a swap with another 12" board, but forget abour the 15s. That board will never fit. Unless you aren't fond of your optical and hard drive. And even then.
Daystar upgrades the 12, just a thought, prob not much cost difference by the time you get the new logic board and DC board and tear the thing apart....
I will second the cpu upgrade. That would give you a good speed boost and having the manufacturer do it gives you a warranty and some piece of mind. Laptops are increasingly difficult to work on with many generations having differing proprietary parts. Not to mention also having very little room to work with.