Today, you can not get the motherboard that is compatible with E521.
E521 uses BTX type and not the ATX type. The easy difference is the angle.
Unless the motherboard is broken, you can't do any upgrade of E521 Motherboard.
Though, some people are willing to sell on ebays. I have looked into Ebay and they do sell the motherboard for E521.
But the price is expensive that I would rather use the money to buy new ATX motherboard and the new case. But of course, that's when E521 stops functioning.