The lens I used to replace my cracked DJI Air Unit Lens is the RunCam RC18G FPV Lens and with how straightforward the replacement was I definitely recommend it.
The first step in replacing the Air Unit lens is removing the old external lens housing. There is an adhesive holding the housing in place so having a heat gun or simply a hair dryer handy is a plus. If you’re using a heat gun make sure to heat up your housing slowly as to not cook the sensitive electronics within the camera module that are right behind the lens. Still be mindful of this if you are using a hair dryer, but I specifically warn about the heat gun because they get much much hotter than a typical hair dryer.
Once you have the adhesive softened up with some heat, take a pair of plier or better yet some Vise Grip Locking Pliers and attempt to unscrew the lens housing. You should feel it break free after not too much force, if it feels like you are breaking your camera module you should probably add a little more heat. Keep slowly adding heat and trying to unscrew the lens until it comes right off and then get to installing your new replacement.
Before screwing your replacement lens into the camera module, first screw the focus adjusting nut onto the threads of the lens, and thread the nut all the way down to the front of the lens. After getting this adjustment nut threaded on the lens threads, begin to screw the lens into your Air Unit camera module body. Get it threaded far enough into the body that it is securely in place, but do not tighten anything down quite yet.
At this point you should power up your goggles and watch what your video feed looks like. At first it is probably super out of focus, this is what that focus adjustment nut is for! By screwing or unscrewing the lens you can get things in focus, and then once the lens body is at the right position you can tighten this adjustment nut down onto the camera module body to lock the lens distance in place, ensuring your feed will remain in focus when you are out flying.