Switchmode or linear wallwart? From the rating (3A 5V) guessing it is a modern switchmode adapter.
Don't mess with neither of those, unless properly trained and very aware of what ya doing there!
The center pin is definitely connected to the heatsink tab and more than likely it won't be connected to ground, but +5Vout.
Whats the problem obtaining (buying) a replacement diode? These should cost just pennies, "1040" schottky diodes are more than abundant, can be even salvaged from an old beaten PC power supply.