if I need to clean electronics I just throw them into the ultrasonic.
May be bad for crystals tho ...
(will do that for the WPC89 pinball cpu board too. Remove the solder stop stuff, put into 25% vinegar acid and throw into the ultrasonic with a pcb cleaning bath.
then start rework - remove bad components (preheat/ hot air/hand desolder pump, flux), clean again. Then reconstruct vias. will be a shit amount of work, but doable.