It's been some time but I decided to finally revisit this. I ordered some adorably tony heatsinks from AliExpress that fit perfectly on the PCX2. I also added a pin header to steal 5v for the fan by soldering directly onto some pads. I applied some hot glue afterwards for some added strength, since this kind of joint is very vulnerable to mechanical stress. I tried to cut away some of the excess material so that it wouldn't look terrible, and I think it came out alright.
I made some observations by playing a bit of Tomb Raider at different resolutions, while swapping around various crystals between 66 and 80 Mhz. There appears to be some kind of frame rate smoothing or cap in this game, since the frame rate always wants to hug 60, 30, and 20 FPS, even at different resolutions. Although this game isn't a good source of frame data, anecdotaly, after swapping the crystal back and fourth, the difference was very clear. Even in the very first room of Tomb Raider, the game went from slightly choppy to extremely smooth. This game seemed pretty borderline at 1024x768 with the 66 Mhz crystal, but was able to easily benefit from even the modest bump to 80 Mhz.
In this test, I found the PCX2 GPU to be stable at 75 and 80 Mhz with no glitches or freezes. I still have more tests to perform and will share more data when I do.