I'm sure that for their repairs they simply replace components anyways. I remember people discussing the topic and it somehow reached a concensus that it was much cheaper to simply replace things that were potentially broken than to pay someone to figure out what is wrong with it, and eventually replace it.

As for Dell acturally selling the mainboards by themselves... I don't know. But if it was sent in for repairs you'd either get someone else's lappy or your own with a replaced motherboard.

And another AC->DC power converter thing would work provided the voltage and amperage is the same.