I'm reviving this old thread to provide an update on my findings:
The Philmore PC74 fits perfectly into the joystick, but resistance is too high for the range of motion of the joystick. This is a 10K pot, and I think 5K or less would be the correct amount of resistance (possibly combined with removing the +/- 5V resistor and adjustment pots since there is a bit of resistance that will always be present on the replaced pot).
I did not have success and in trying it out damaged the joystick too far to continue, so I'm keeping the controller for spare parts now.
In other news, I replaced the polystyrene caps for the deflection circuit with MLR472K100 and MLR103K630 and they worked just fine.