That does look pretty crusty. If you have free time give it a go cleaning it up. I wouldn't mess with looking for a working original board to put in if you fail, just get replacement boards of your favorite flavor (pascal, niwumpf, lisy) and replace.

Make sure you update the driver board with protection diodes:
https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_1#Driver_Board_Issues

If you decide to replace the whole set of boards with the pascal x4 don't bother with the power supply or driver boards.