Reflow solder on all of the display header pins as the solder joints are usually cracked. Do this by removing the old solder first, then adding fresh.

Do any of the playfield coils work when you start a game? I’m guessing you also need to rebuild all of the connectors on the rectifier board.

Pinwiki has a good process for bulletproofing these games.

With Dolly Parton, when you start a single player game the player 2, 3 and 4 displays go blank and this is normal. Press the start button again for a two player game and the player 2 display should come to life showing '00'.

Anyway it sounds like you have some connector issues. It will help your cause if you post some pictures of the boards in the backbox.

I think you have 2 different issues.
Since you worked on EM games I assume you know how to solder? Your going to find that these old Bally SS games many of these problems are cold solder connections to the connector pins on the boards. Best place to start is to reflow/resolder all the connector pins on the boards.

Lets start with the flippers as that is the easier to troubleshoot. Since the other coils on the playfield work The flippers are probably tied to bad connections on the solenoid driver board. That's the board to the top right of the game. Check the connections on J1-3. and the relay it'self. Reflow those and your flipper relay should kick in and you'll have flippers.

As for the Displays,
Reflowing the connections might help. All of these connections trace back to j4 and j1 on the MPU so as a last resort you should check those connections.

Finally post some photos of the game and boards. If there is a battery still mounted to the MPU, remove it. Those leak over time and damage the board.

Good luck, awaiting your results...

When the game starts the flipper relay on the driver board should make an audible click. If you cannot hear it probably enabling issue related to connector MPU J4 Pin 7 to driver board J4 Pin 8. Examine back of board for cracked solder joints. The MPU J4 gets ate up by leaking batteries and cracked solder common there too.

If you can hear the flipper relay click but still no flippers check driver j2 pin 2 & 1 and driver j1 pin 8 & 9.

The displays themselves are single sided boards and the weight of the wiring harness and the when the door swings open it flexes on the connectors and eventually the solder joints crack. It is super common on these games that every single display will have bad solder joints from 40 years of flexing. I would start with making sure each display has good solder joints. If still have problems with displays then check out connectors at MPU J1.

Anyways.... connectors connectors connectors. usual problem. Its a pain to repin all the problem connectors in a game, but needs done in a lot of project games.

Be cautious around those scoring displays when they're on. They'll shock the crap out of you. Your rectifier board connectors definitely need replacing. Get a crimping tool if you don't already have one. I bought this one off of Amazon and it works fine.

https://www.amazon.com/HT-225D-Cycle-Ratchet-Crimping-interchangeable/dp/B007JLN93S/ref=sr_1_1