May be the wrong way to go, but when I painted my tractor.
I drilled out the plate attachment rivets then had the plate so I could dip it in a can of cleaner for proper cleaning.
I reattached the plate with small screws, however, top restorer might not want an I'd plate attached with screws.
Main thing is take your time, try to get the old paint off without a wire brush or scratching the surface.
Might go to auto paint store and ask them for suggestions if you want it to look good and save the surface.