I don't really care, not planning on dying within the next few decades. Maybe by then, we'll already be in space, and whoever has to make that decision can just push my body out of the air lock.
You can never truly be prepared for death, natural death just like accidental death can happen at anytime in any place with out you knowing until it’s too late.
Not sure about being registered as an organ donor because it’s different for every country, maybe try looking up what to do for your specific area and see what comes up.