You probably need to be more specific in what you actually need. There are lots of ways that people make a game "free play".
In the case of Sky Jump, it can be anywhere from someone adding an external push button, rigging the coin return button to add credits, or the proper way, bending a contact on the Replay unit to fake the game to think there are always more than zero credits. Reversing the information located at Pinrepair.com for EM repair might help you.
For the Bally SS games you listed, free play software/roms exist that keep the game at 01 credits or make it still start when on 00. That would be nothing more than changing a dipswitch. But you would have to know what software your games are running. Or, sometimes, people wire an extra switch behind the start button switch or rig the coin return hardware to trigger more credits.
And none of this addresses if and what might be missing on your coin doors from as little as the coin mechs (slug rejectors) to as much as the entire guts having been removed so there is no where to install a coin mech.