Can I replace the motherboard on an iPhone 4s?

A friend of mine dropped her iPhone several times and as a result the front screen is smashed and she's been told that the "motherboard is cracked" (her words). She's given up on the phone ever working again and didn't want to fork out the cash to repair it so she'll be giving it to me (because she knows I like to tinker with things).

Is it possible to replace the mother board in an iPhone 4s? I know it's possible to replace the screen and I'm prepared to put in the time and effort for that (I realise that it's a big job).

Also, I may as well ask whilst I'm here, is it possible to replace the logic board? It is possible that this is what she meant when she said "motherboard."

Thanks