No need to disable the coin switch, just wire the button up in parallel.
Most games have a free play option, but if this one doesn't, a credit button might be a necessity.
You don't need to drill holes in the cabinet to mount it either. You can mount a small button in place of the lock cylinder - either use the above lock switch, or mount some other small pushbutton in the hole where the lock would go. To get the coin door to stay closed, you can simply bend the door slightly, or put a piece of tape in it so it'll wedge closed without the lock.
Another option is to mount a set of leaf contacts such that pressing the coin reject button closes them.
You can use any sort of momentary contact switch you want, leaf or microswitch, it doesn't matter at all.
One trick, which doesn't require adding a button or a hole to the cabinet, is to run one wire from the coin switch (the signal wire, not the ground) and connect it to the signal side of the 2 Player start button. It doesn't work on all games, but it works on most. That way, you press the 2P start once to add a credit, then press 1P start to begin a game. Press it twice to start a 2 player game.
-Ian