So it sounds like you may be trying to repair an MPU in a Star Trek that may not be broken.

While the MPU could cause display problems, it may not be the cause... I apologize if you know this but a single blown/bad display can affect other how ALL displays work in a Bally -35 game. I would start by testing each of the 5 displays individually in the Star Trek.

The output signals PA4-PA7 from the U10 PIA are used to select different numbers to be displayed. Are these signals pulsing at the displays (pins 19, 18, 17 and 16 at the display connectors)?
You might have bad connections at the MPU J1 connector (pins 25 - 28 respectively) where they exit the MPU board.

The U20 chip is involved in selecting between displays. U14 and U19 are for the blanking signal. These are unrelated to the symptom of only showing 8s and 9s.

First check snyper2099 suggestion above of determining if there's any bad displays. Then do the logic tests on the PA4-PA7 signals which contain the display BCD (Binary Coded Decimal) data.