There’s a “coin unit” that advances through the 4 players. It’s a stepped unit on bottom of cabinet. It probably needs attention (gummed up so it won’t advance)

In this market $900 Canadian is in the ballpark. Depends on condition.

For your issue, is the problem that pressing the start button doesn’t add more players? Or that in a multiplayer game, it doesn’t advance past player 1.

That’s the Player Unit, which routes the score etc. as it advances.

Look on the schematic for the Player Unit Step Up coil. It goes through 4 or 5 switches to make the circuit, you’ll need to inspect all of them.