With power off, adjust all of your score reels to something other than zero. Power up, try to start a game. Do all the score reels go to zero? If so, and the score motor is still running, look at the "Z" relay(s) while the motor is still running. Is/are it/they still energized?
I am getting to a point where I can't help much without a schematic. I'm not even sure if the score reset relays are labeled "Z" in this machine. The only machine I have close to that era is a Sing a Long, and it does not have any relays labeled "Z". Do you have a schematic?
Part of owning a machine is the learning curve required to fix them when something is wrong. I know its frustrating, but once you repair one and get it going, there a large feeling of satisfaction. Plus, you can report what you found and did to fix it, and possibly help the next person with the same problem. Hang in there, the problem will be solved!