I think you are on the wrong path, the U21 pin you were looking at is part of the RTC and isn't needed for the game to operate. Did you try swapping three ROM and CPU with another game?

This may help determine pathways between pins.

Note REAR side is inverted to help with pin-to-pin checks.

Good luck.

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that pin doesn't have a digital logic level, you won't be able to check it with a logic probe.

I have personally never seen a crystal oscillator fail and the entire oscillator circuit you are referring to is made up of passive components, the likelihood of a failure in this area of the board without physical damage is infinitesimally small. Make sure you have your logic probe set to measure CMOS if it cant auto detect and that you are using 5V to power the probe, with my probe I can see both high and low lights light solid to indicate I am measuring a pulsed signal.

Have you tried replacing/reseating the ASIC chip?

Mine reads both high and low at the same time when checking R99 (either side) ... the best way to check this is with an o'scope. FWIW, this is what I have across R99 (ASIC 54 - 57) and the frequency counter reads a solid 8MHz

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Logic probe at R99 ...

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Test equipment error. There is no way for this oscillator to not pass through 0V, the positive and negative polarities are going to be the same levels in opposite directions.

I would make these same checks on a known working board with the same logic probe for verification. I would also confirm pin 1 of U10 is NOT low, if it is, the ASIC is being held in a reset state.

I would also try a different ROM in the board.

Yep, that's what you should have.