What I have done in the past is use one of these micro switch holders from newer games - https://www.marcospecialties.com/pinball-parts/545-5121-00
The mount holes are the same spacing as those that hold the leaf switch for the start button. Bend the actuator from the micro switch so that it is activated before the start button leaf makes contact. Either wire the new micro switch to the coin switch of just use some alligator clips for a temporary free play setup.
Each time the start button is pressed, it'll add a credit and immediately remove it to start the game.