Here is a video about Core Strength Exercises. There you may find something that is good for you. Good luck!
(Sorry I posted this twice, I thought I was answering and was commenting. technology is not my strong suit)
It is useful when working though any pose to pay attention to mulha bandha and uddiyana bandha. The belly and the root lock help to create intraabdominal pressure and a lift through the pelvic floor muscles that help to protect the back and allow the line of energy to open in both directions.
You need to consider the breath, the placement of the body in relationship to gravity, and which of the vertebral joints have the greatest/least movement. And remember though asanas such as navasana have a strong abdominal component, they need to be used in the context of other synergistic postures. That is one of the ideas of flow yoga… that the interstitial space between the postures matters as much as the held postures.
I teach core yoga every Saturday, and if you are ever in RI you could come try my class to see my take on it.
I would like to recommend to you Mel Robin’s book “A Physiological Handbook for Teachers of Yogasana”. It is an excellent book, though one that I think is best read in small chunks as it is quite dense and thorough.