Sorry I can't really help with your question because I don't have either of those kits yet. But you could try contacting Forge World and see if they are kind enough to send you a replacement head. It's an expensive model after all so some goodwill isn't too much to ask for
Having just built my Y'Vahra a few weeks ago and having built a couple Riptides over the years, hopefully I can give you some help. The standard Riptide heads are slightly smaller than the Y'Vahra head, and don't really mesh as well with the more sleek design of the Y'Vahra, but they would fit alright in a pinch. I would also recommend contacting FW customer support and seeing if they can help, they're generally pretty good about that kinda stuff.
Glad to hear they did, once it is done, don't forget to post project log, or join motivation service post so we can see the goodness, also it helps getting your models done once you start getting lazy/procrastinate about it