As a normal precaution, I disconnected the motherboard from the PSU and when powering it up, every fuse on the board blew. All three large capacitors were shorted.
 

The large capacitors have 4 legs, middle pin is positive (+) and the outer ring legs are negative.
 

Positive pin marked with red, and negative pins marked with green.
You can replace these with a standard 2 leg radial capacitor (r=11), just make sure the negative planes have continuity.
 

After a full recap and replacing the rectifier bridge (with better specs to run cooler) the PSU powered up and gave stable +5/+12/-20V.

 

Wires from the transformer to the PSU PCB were brittle and snapped off just by touching them.
 


New solder, shrinking tube and a zip-tie to the rescue.