Dust kills power supplies. After this, you'll change them. Look at a working version of the supply, if any are available. Get a schematic if you can. Apple and Sun are lousy for schematics, HP is good.
Clones are impossible.
Look for all the usual visual stuff, open traces, shorts, burned components. Don't use the regular circuit as a dummy load, though you may want to find out how much current it draws, to help you build test loads. Use a bench supply for testing the regular circuit. Look for shorts. Look for ESR or opens.