Are there any natural bowel preps I can use before my colonoscopy?

Yes!! Senna, an ingredient in many laxatives is a vegetable product. There are several non-absorbed sugars that give a clean prep without any "chemical" we often use sorbital- an ingredient in "sugar-free" candy. Diet is important for prep, also.
Not really. All bowel preps work by drawing water into the colon causing liquid stools. Most natural agents are stimulants and only promote some evacuation of stool and can cause cramping. Magnesium citrate is the most "natural" prep that can be used to insure a thorough cleansing of the colon.