It does look quite grungy.
No idea where you can find schematics, but a ZD designator in a power supply has got to be a zener diode used as a reference voltage on the base of a bipolar power transistor - if you can figure out what power rail the transistor drives, you can probably just select the zener of the same voltage to drop it in.
Between the grime and the conformal coating, though, I don't envy the repair job...