Camera from flea market with bad corrosion

@andrew27242 it would be nice if you could see what you see, so we can be way more concrete with our answer. You can post images with your QUESTION. Just edit it and add those Adding images to an existing question - iFixit Repair Guide

In general, you can remove the corrosion from the battery by simply using a cotton swab and vinegar. Use that on the contacts etc.

If it is rusting on the metal parts vs. the blue looking corrosion caused by the alkaline battery, then get the smallest can of rust remover at your local automotive or hardware store (Rustoleum and Loctite have a gel consistent remover that would work great). Use a cotton swab and apply it carefully. Let it finish its chemical reaction and then clean it with isopropyl (90%+) and a cotton swab.