Okay, a few things to check. That credits switch that they "added" make sure the two black wires aren't accidentally touching all the time. Same with the blades. That switch should be open until you press it. Can't tell in the pic, but the soldered ends look very close together.
Next, the tilt plumb looks like it's "made" to me. Wrap a paper towel or something so it can't touch the sides for now, just in case (or just remove the plumb bob). the Rolling ball tilt looks fine, but the slam switch, I can't recall if that should be normally open or normally closed. Try starting a game both ways.
Is there a slam switch in the door? Try the same, both ways. A slam switch under the playfield? Etc. Make sure balls are in when you try these, but a game should start up...
Were you able to try the test/diagnostics button yet to see if there's any luck with that?
Sorry I don't have the schematics in front of me to see if the tilts are NO or NC.