When it comes to legacy Bally games you can freely use LEDs for the GI lighting without any board modification. In fact, I highly recommend replacing GI lights in all games as it reduces heat and power stress and helps to preserve sensitive elements like the backglass, plastic, and rubber.

As for the switched lamps, that can be argued both ways. Non ghosting LEDs like Comet sells can often be freely installed without board or resistor modifications. However in some instances even non-ghosting LEDs can have noticeable flicker. My personal opinion is where bulbs sit unexposed benefits of LEDs are minimal and to leave them as incandescent. In other locations, such as where the bulb is exposed or behind the backglass, LEDs have sufficient benefit to be worth installing. If you want to experiment it would cause no harm to try Comet non-ghosting LEDs and note if you observe noticeable flicker before performing any mods.