Nipple stimulation! You can actually help regulate contractions naturally without meds sometimes this way too! I did a lot of "cat" exercises right before my water broke too. Get down on all fours and arch your back like a cat and rock back and forth like that. This encourages baby to move downwards to put pressure in the pelvic area which helps the cervix dialate. You can do pineapple core too.
I wouldn't try it. The castor oil can cause the baby to inhale meconium stressing or even killing the baby. I had a friend who lost her baby because of it. Have you tried other natural ways?