Some people only use the thick white portion of the bok choy. I am a cheapo and use it all. I hate to waste any of it.
Sea salt can be substituted for the pickling salt. If this ends up too salty for you, just adjust the level of salt on your next batch.
Cayenne pepper or half cayenne pepper and half red pepper flakes can be substituted for the Korean chile powder. Wear some gloves if your skin is sensitive to red pepper or spices.
Jalapeno peppers can be substituted for the serrano pepper.
The nutrition data for this recipe includes the full amount of sodium for the salt. The actual amount of the salt consumed will vary.