Re: Tub Overflow
The gasket is replaceable, if you can't find the same size, then you can purchase gasket material and cut your own. As far as the rust goes, unless it's significant, don't worry about it. If it is significant, the only way to remediate it is to remove the rust and repaint or enamel the hole to minimize the rust from returning.
When you replace the gasket, wire brush as much of the rust off as you can, then reassemble the overflow.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!