Repair the switch - one leaf is indeed broken. Clean the contacts for the score reels and the relays that control the chimes - I don't remember the designation, but the relays are in the head.

Since the previous owner has been playing with the score reels, that would be a good place to start looking for issues. Let us know.

Forgot to mention how to clean the contacts - drag a business card through the closed contacts to remove tarnish. YOU should not need anything heavier (Sandpaper is a definite no no). Verify the screws that hold the switches are tight - closest to the contact first, then the back one.

If the switches are riveted together, you want to make sure that both rivets are intact and sandwiching the pieces.

You are going to want a schematic - Steve at Pinball Resource sells them. Gottlieb schematics are generally not available on line, and they actively keep them from showing up on line.