Agree here with @Moo and @PaulL. Salt, salt, salt, and more salt. When I slack off on my salt intake I get cramps in my calves (only at night 
) and if I continue to slack, I get cramps and muscles aches all over the rest of my body.
Also, keto is anti inflammatory. You may find over time your knee starts to feel better. 
I have torn menisci (both knees). I learned that lower body exercises can help. Leg and also hip exercises. They might not solve the knee problem, but strengthening the stabilizer muscles means the muscles take some of the work off the joints, which might prevent further deterioration.
I had a bunch of floor exercises and standing exercises for this. They did help, but they were boring.
Now I have a rebounder (aka expensive mini trampoline) and do a variety of low-impact moves, e.g., stepping front to back and side to side; marching with feet together and feet apart. In my opinion they cover almost the same bases as the targeted exercises I was doing before.
Every day I do about 20 minutes of YouTube rebounder videos and 20 minutes of rebounding to music (in between other exercises). If you do the videos, you might have to modify some of the moves so you don’t aggravate your knees.
As I mentioned, I just step–no jumping! But that’s me. I’m female and much older than you.