A regular chemical paint remover is probably the best bet. Let it soak a bit then wipe it down with a rag. Repeat till numbers become visible. A brass wire brush can be useful too, but don't sand on it, abrasives can obliterate the numbers rather quickly.
If you have paint stripper, check the frame also for the frame stamp. Strip the paint from the frame stamp and if you still can't read it. Hit it very lightly with some very high grit sand paper. If you use about a 400 or more grit it should just shine up the frame and leave a little junk in the letter and numbers making it easier to read. I'm not sure this is a good idea for the plate though because I don't think those numbers are stamped as deep.