The chances of a Singer 99 becoming collectable (valuable $$ wise) in our lifetime is slim. They weren't the top of the line sewing machines and tons of them were made. I know they often say that antique furniture should not be altered, as it affects it's value. I don't think this would fall in that category for three reasons, it's not furniture and the two reasons listed above.
To illustrate my point, how much is a beat up old black featherweight worth? How much is a pristine shiny cherry apple red Featherweight worth? How much is a perfect black featherweight worth? A white one? A 222?
Shine up or fix up your case. That way you will enjoy it more. It won't make any difference in the value, unless it enhances it.
JMHO
bkay