Like the other person said, better off getting a new phone.. The "motherboard" of a phone is pretty much the whole phone minus the screen and keyboard (if it has one). But before you do that maybe some trouble shooting could fix it.
The question is if you can find one. I would try to find a phone on craigs list that might have a broken screen but no liquid damage. And you should be able to take the phones apart and replace the item needed.