Don't try to get it to work in play... get it to work in test first.
Check the playfield glass switch (upper left corner). If that's not working, the MPF shuts down so you don't trim your fingernails.
After that, it's most likely a connector.
Start at the motor and work out.
Check the solder joint on the motor lugs
Then check continuity on the connector wires from the lug (not solder joint) to the connector tip... c/b a wire break.
Then check continuity from the other connector to the bi-directional board.
Then check the connector on the black/red wire. It's 25 years old... Give it a good squeeze. check continuity again.. from the bi-directional board to the lug of the motor... if you're still good check for power going into the bi-directional board.
Here's a few things to try, easy to hard.