beautiful flash hider! I'm excited for you on this build.
My recommendation for this MGC is to start light, to see what you're working with. Maybe a toothbrush and a paste of baking soda and Dawn dish soap, 0000 Steel wool buffing or a light/quick polish with Brasso or something. Test in small areas, or the inside of the magazine plate to see if it removes the finish, I don't think they will.
Maybe you'll get rid of a layer of corrosion or gunk and see what color the metal is without other stuff in the way? It does look like the lower frame took a heavy beating...
I also have a syringe of this red gel gun grease that I squirted around the bolt and action inside since I basically rubbed off the old grease they used when i disassembled and cleaned my gold ones