A little more information would be helpful, like the name of the game and the problem you are having. I'm guessing it is a Gottlieb?

Rottendog made a sys1 replacement?

The idea behind the system1 ground mods to run common ground wire all to that grounding panel in the cab. So just find a good spot on the MPU's ground plane you can easily solder a wire to run to the ground plane. Be neat when doing this. You can make the ground mods look really hackish if you are sloppy.

The Rottendog power supply board doesn't need a ground mod. I'm assuming any other modern replacement board wouldn't either. All the Gottlieb boards need extra ground wires though.