I have several stern/ballys from this era I have found it is much easier to just get a repo MPU board and be done with it when it comes to MPU alkaline damage. The time and frustration saved is worth the 200 you will spend. That being said your power rectifier board doesn't look to bad and could easily be fixed. Also repinning the connectors on your MPU and power driver board will also make your life easier as well. Good luck my Tident was my first pinball I played and have had one in my collection for several years it is a great game...
Good luck with it.
Al