You can try, but the odds of identifying the failed component(s) visually is pretty low. Attempting to trace the energized circuit using a multimeter is extremely dangerous, for obvious reasons.
Fortunately, the PowerMac G4 series use power supplies which are very similar to those commonly found in home built PCs - you may be able to find an appropriate substitute on the shelf of a computer store which caters to the homebrew crowd. As long as the molex connector to the motherboard matches, and the supply wattage is equal to greater than the original supply, you should be fine.
The link you provided seems to point to a resource I personally would take advantage of - the price seems right, especially considering he's done all of the labor and testing!