I have an iPod mini project I am currently working on. Usually, a Sad face error message either means that the motherboard is fried, often due to water damage. I strongly recommend replacing the battery as they had a tendency to fail after a few years, and then if the issue still isn't fixed, consider buying a donor iPod. It is virtually impossible to find a factory spare motherboard, as they have pretty much run out, or cost more than a replacement iPod. If you can find one with a damaged screen or a scruffy shell, then you could get it fairly cheaply. I picked one up just today for my project for £10, which i'm going to take the motherboard and top clips from. I can post pictures up if it would help.
~Dishwasher97