That's not a metal plate. it's heat resistant mica composite. The microwaves come into the oven through the hole behind it. You can take it out scrub it off, put it back in and be good to to go. Be gentle with the stuff. It'll crack if you scrub too hard. From the soot marks on the metal above the piece, you must have had a big blob of grease explode onto the shield, where it burned merrily. Clean the soot off too. Pull the piece up gently to remove, and push it down under the clips to put it back.
I have always been told by my dad that a fire in ANY electric appliance warrants an expert who is knowledgeable in the repair of the appliance before it is used again. Electricity is one thing, microwaves are another. All of the shields are there to protect you and your family. What price will you pay later to save a few dollars. My advice is take it outside and destroy it. People go thru dumpsters to pick up stuff and sell it to someone who is trying to save money. If it works use it is bad advice. But do what you want and I hope you sleep well