In order to do this right you will need to remove the metal parts before you can clean and polish them properly. Consider first, are the parts solid metal or are they plated. If they are plated the best you can hope for is removing the rust and smoothing the rough areas, unless you want to go to the expense of having them replated. If they are solid metal there is a simple method which I use and it works very well. A product called BAR KEEPERS FRIEND, a few rags and a little water is all you need, (a little very fine sandpaper, 400-600 grit if the parts are really rusty.) Start by mixing a paste of the powder and water, use this like scowering powder to clean and remove the rust and smooth the surface. Once you see bare metal, start thinning the mixture with more water. As the mixture is thinned it changes from a scowering cleanser to a polishing compound, the wetter the better for polishing. This takes a lot of rubbing so have patience. When you have restored the "shine" rinse the part with water, be sure to remove any compound, then dry the part, use a blow dryer, this wil dry and heat the metal. When it is completely dry and slightly warm to the touch spray a thin clear coat of acrylic or laquer on the parts to prevent them from rusting again. Let them dry for a couple of hours then replace them on the amp.
*afterthought* For the finishing coat of spray, hang the parts on a thin wire so you can coat both the inside and outside surfaces. An old guitar string through a screw hole works well for this. Also a Dremmel or similar tool with a felt buffing wheel will make this repair a lot faster.
