Yep, it's the (Italian!) add-a-ball variant which means it won't have a credit counter and that a single coin drop activates the start relay.

(It's plastic)

That would be fine, you just have to make sure the voltage rating is quite high. Back then I'm not sure if Bally used 120V or 30V through the replay circuit. You can probably find one with like a 600V rating quite easily, that may even be the standard.