Twin Cities, MN
For Gottliebs that have a standard credit unit to award and track credits and replays, there's a simple, reversible mod that can be made that:
a) puts the game on free play; and
b) resets the credit unit to 0 at the start of each game.
This might be interesting for folks who would prefer replays to "mean something" in home environments (they do, but that's another discussion.) 
Anyway, the idea is that at the start of each game, the credit unit is at zero. The real objective for beating the game is to achieve specials, or replays. So consider this as a 'points' system. As replays are won, they're worth a point, and the most points wins. In the case of a tie, then highest score wins. Since the credit unit resets to 0 at the start of each game, there's no need to remember what it had been, and the credit unit has the full range of 'points' available to score (typically 15 or 26, depending on era of game).
The mod is super simple. To set it on free play, make sure the middle switch is always closed. This is the switch that detects if there are credits available to allow the start path to work. The switch can be adjusted so it doesn't open when the unit is at zero, or a small jumper wire can be used, etc.
To set it so the credit unit resets each time, just flip the top ratchet arm up and over (counterclockwise), so it no longer 'catches' the cam on a decrement. When a game is started, the Subtract Replay coil will energize, pull in the plunger, and the cam will zip back home, since the ratchet isn't there to stop it.
To undo, just flip it back in place, and adjust the middle switch back to open at the zero position.
[Edit]
May also want to consider moving the Match feature In/Out adjustment plug to the Out position, so a Match credit isn't awarded at the end of the game. Although it does offer an element of chance for a come-from-behind victory.
Here's a picture of a credit unit, annotated (in this picture, the ratchet hasn't been flipped over, but you'll get the idea).
