Well, I don’t know how to do a proper job of it, but I know a way to fake it and produce a functional flight model that you might like.
Find a plane you have with the same number of engines and about the same performance and with the same kind of landing gear (tail dragger or nose wheel) copy the aircraft.cfg and air files. Delete the User Interface section in the cfg file and replace it with section from the plane you’re modifying. Don’t forget to change the air file name to match the new air file.
Then copy these sections from the old cfg file and paste them into the new one:
Contact points
Lights
Effects
Smoke System (if applicable)
and any other sections that are appropriate. Alas, I don’t yet have a real computer so I can’t bring up the cfg file from a model to see what I don’t recall right now. I don’t know if I have actually forgotten any.
Save copies of the original files then put the new ones in the model folder and see how the plane flies. Hopefully it will fly more to your liking than the originals.
If nobody has a proper set of flight files, this might get you airborne.