Since the part that touched metal is a heatsink, not an active electronics part, but it still arced and burned it, we'll need to know what that metal that was touched was wired up to.
As much detail of exactly what the situation was, and exactly what happened, could be helpful.
If the heatsink is not electrically isolated from the parts bolted to it, then those parts would have had current flow thru the heatsink to whatever it was it touched and arced to, and those parts could be damaged. If you want to do a "blind" repair you can just look up the part numbers that are on those parts and buy new ones from Digikey, Mouser, or Farnell, for instance, and see what happens, but for now there is still not enough info to help you fix it.
Other than that, I can suggest again that you measure voltages of the circuits from output back to input and see what you read, not *just* the output and the input, and document what you get at each point (perhaps on a copy of the pictures of the device).
If you can't or wont' measure at the parts inside, you will not be able to test, troubleshoot, and repair this unit (or anything else).
