you should be fine

I can't think of major problems in your fasting straight from ketogenic diet. It is possible that your elimination would be less effective, depending on how much fiber you consumed, my understanding is that ketogenic is not as high in fiber as plenty fruit-veg diet would be. Not sure what would be also the vitamin-mineral reserve of your body. Although, I believe you should not have any major disadvantage compared to the usual faster here.
Best,
DL