Nice try, but no soup for you. Organ 'donation' is a consent arrangement; in the case of a living donor, the alive individual consents; in the case of a deceased donor, the person is no longer alive and the family decides or the once alive individual's donor card decides the issue. You favor taking organs from alive humans?
An unborn fetus, whether through abortion or miscarriage provides those cells and I think we should use them. I don't see your distinction between organs and stem cells. If you are opposed to one on ethical grounds you must be opposed to the other. Your excuses don't fly with me.