I have had a great deal of success with popping the switch out at the joint and then hitting the pieces inside of switches with steel wool. Replace the springs with common springs found in pens and boom, you are good to go. This has been the issue on most of the old "Broken" Power Wheels that I have been working on the past month.
You mentioned the terminal is broke .... I've never had these apart but look a lot like the aftermarket switches at your local auto parts store ... Might be worth a try I don't know about momentary but maybe you could soder in a new terminal from the 3 dollar doner