You actually have all the parameters needed - the real model you are flying!
I'll be brief, as I've typed longer "how to's" previously, which with some digging you should be able to find.
Find an existing model that has the same look as your real. Doesn't have to be exact, and doesn't need to be the same size. Nicer if it has the same kind of power (electric, glow, gas, etc.).
Make a copy. Then find the settings to increase the graphical scale, so the wingspan is the same as your real. If flown, it will fly very badly.
Go into the aircraft editor, and start changing as much as you can to match your model. Part dimensions, weights, airfoils, positions of the components relative to each other, etc. It will take a bit to get familiar with the coordinate system. Yes, there will be a bunch of settings that you may not know what they do. But give it your best guess, or keep what is there if you are really confounded. I do this pretty often - back and forth from the plane to the computer with various measurements. You can add any kind of component (extra wings!), or take away any component that is not on your model. For part weights, estimate, and then use the overall adjustment to get the model to end up being the same as the real. Test fly, and adjust CG for best flight - due to your estimating, it may not end up in the same place as your real model.
If you can get most of the main part sizes, and the placement of the major parts, it will fly very close to the real thing. It will still LOOK like the original. A redone color scheme will help hide that fact.
Without a detailed reference, as you noted, any other person trying to design it would not have access to as much information as you do, unless they also owned the model. So while they may make a nice flying plane, it may not be any more "real-life" than just using one you found that is already done and sort-of-close.
I've actually used my self-created versions to test various fixes if the real is not flying right. And many times after testing in RF, they work very well in the real model!