nice pick up of gear, baby steps first and then you'll be posting on how to fix issues 😉
an Altek MPU board also has a processor and the 2 PIA's U10 & U11, but I'd really only swap them after swapping out the SCR's if you've tried joining from TP3 to the Gate and they don't light.
just for practice you can remove the suspect SCR and replace it with a known good SCR, just to get soldering and desoldering a little work out?
pretty sure you mentioned trying this and they didn't light, but to prove you are doing it correctly touch the Gate/s of known good bulbs, they should light, if not then there's an open circuit.
i don't believe a multi meter will pick up the output of the decoder but you can try and compare it with output legs of known working SCR's if your keen?
a logic probe should and an oscilloscope definitely will.
first start ensuring the globe holders are in good shape, then continuity from the globe holder to the relative SCR which is done by using TP3 to the Gate, then test the output of the relative decoder.
it would be unlikely a PIA is at fault, but in pinball as in life, expect the unexpected.
Good Luck.