If the case/heatsinks are aluminum, you can use very fine sandpaper (I'm talking 600-800 grit to start, and work all the way down to about 1500-2000 gtit) to sand out the scratches, then use an aluinum polish to buff everything out. This will make them very shiny though, not "brushed." (I did this with my Kirby vacuum because it was getting all scratched up and now it';s like a mirror)
You can take the guts out of the amp, and send the aluminum case off to have it anodized in nearly any color you could dream up.
you *could* just spray paint it, but this won't come out very well, and the addition of paint of this type won't adhere well to the aluminum without dipping the entire case to strip it of any foreign particles, skin oils, etc.
Aluminum can't be powder coated because it's not a conductive metal (ferrous) but if the case is a steel alloy, this is another option.
Lastly if it's a really good amp, you can do what I've been considering for my Orions, and have the amp case gold-plated after sanding out all of the imperfections and scratches, then having the name markings etc re-engraved into the amp.
Oh, you could also look at having the case for the amp chromed, but triple plate chroming is expensive, like gold plating, and it will affect the heat dispersion characteristics of the amplifier.
we used to do that with audiocontrol EQs and signal processors. We'd get the white ones, make a template of the printed stuff on the case, then take it apart, powdercoat the cases different colors to match the car interiors, then re-silkscreen the device faces so you could still see what all of the controls did. If usually garnered a few extra points at IASCA events.