Just found a nice PET repair video by the 8-bit guy here:-
https://www.youtube.com/watch?v=uHbhH7ISL_Y
The section at 2:42 onwards is of particular interest to you because it has a nice explanation / animation showing how the keyboard is scanned, and the various methods he uses to narrow down the fault.
Although his fault is different (no keys working at all) the diagnostic process still applies. At the moment my money is mainly on the 74LS145 decoder being faulty on yours since you've already changed the 6520.