While cleaning my camera, this piece was scratched. What is its name? and is it possible to replace it?
That's a 'focusing screen'. You can easily obtain it either from the manufacturer directly, or from a camera store.
For example from Adorama
Typically, for one model of camera there will be a few types of focusing screens that work with it. Your camera's manual will tell you which one has been installed by default, and which ones you can substitute for it.
Manufacturers can also are helpful: Canon's guide to choosing a focusing screen
The only problem is that for a piece of plastic that they are, focusing screens are pretty pricey.