I will add my thoughts. I speak from the perspective of one who is older and has lost my parents to death.
I would buy a beautiful engagement announcement card and send it to them. Mention nothing of the past, except maybe that you always dreamed of finding someone to share your life, and now you have. (perhaps if you as a child wanted your mom to marry you, you could add that) You could add that you'd love for them to meet your intended. If you hear nothing, you know where you stand.
To do nothing, however, could mean regrets for you later. While there is life, there is hope. Or, as the Bible puts it, love never fails.
P.S. When the day comes that your parents can no longer care for themselves, I pretty much think that they or the elders will be contacting you, reminding you of your "family responsibilities" -the ones the elders will not want to assume.