I'm not sure I have a spare. I have just put a thread together, to sell all my stuff, since being in a motorcycle accident, I'm not going to be able to continue rebuilding and playing with my 787's. You are exactly right, these are only called dream-o-meters. I actually have a motorcycle case for my iPhone, with the ROKFORM case to help protect it from the elements, and I use it for navigation. If you'd like to know your true speed, go to your app store and download "Pro Charts". You'll be surprised. It's like a mini garmin, gives you location, accurate speed, and you can actually use it to plot a course, setting waypoints. I love it.
As for the speedo, if I remember right, you'll need basically the same type you are using, because it works by magnets in your paddle wheel. I think, the other has 3 wires, and it was for the temperature. So yes, if you pick one up that looks like the one you have, but extra wires, then determine which is the temperature, and which is the speedo. But honestly, I'd had my watercraft a long, long time, and I've used Pro Charts for as long as it's been out. It's also great if your in a river your not familar with, up creeks, it'll keep you from getting lost.