I see you are already informed on the rubber gasket; I did the same. I graduated from kindergarten, so i mastered the skill of scissors long time ago. Besides the Amazon link and a few dollars, it's all you need to re-make the gasket.
For the metal plate painting is a good option. Should you want to go professional, these were originally bright zinc plated. Google for shops near you.
Often when you have small parts (bucket of nuts and bolts) they work by a batch principle as they chuck it into a tumbling machine that processes it per batch.
Whenever you have lager parts they hang them on racks, dipping into various baths. Then you pay per item, usually based on the amount of surface area.
The cleaner the steel, the better the outcome wrt corrosion resistance and uniform looks.