Here we go again...

[PieEater]

You won't be able to buy a blanking plate you'll need to cut a bit of sheet metal to size and drill holes etc or find someone that can do it for you. Try leaving the valve assembly soaking in penetrating fluid for a couple of days to see if that helps, you might be able to use heat to free it up as well. Be careful though as it's easy to bend the valve out of shape if you manhandle it too much.