I have never taken a 3G apart. Don't get to play with too many iPods. So , no idea I'm afraid.

Yes, I know. That was really helpful.

I will offer this: Hard drives are not worth repairing. If you want to fix one by swapping out parts, you need not only the same make, model and size, but for some parts even the factory, batch and firmware must be matched too. And anything mechanical must be done in a clean room or you will destroy your platters in no time.

As for mixing and matching iPod parts, you might well get away with a different drive, but I have no idea about things like click/scroll wheels and the various cables and connectors used. I would expect a fair amount of incompatability though.