Hello Walter, is it Walter? At least I remember you as that.
The overlays are usually made when the model is built and essentially take a section of the already mapped and coloured area that the insignia would be placed on, it is a duplicate say of the area of a wing, fuselage etc. There are entries in the model's plane folder for these called overlays that use a mat file to allow the markings to appear in that area that you make when building the aircraft, so it is likely you would have to make new ones sorry to say. The overlay is actually a section of the model that is cut out and made to float shall we say above the actual area where those markings are to appear, they are not a generic thing as such and are model dependent.
For my Lysander I actually added these as meshes into my plane folder, I do not think that is the usual way, normally these overlays are part of the actual model itself and are saved as a nul file or something when you make the aircraft so that they can be activated by the mat file.
It is a long time since I did that stuff so I could not say other than what is above, really I think you would need to create those again from the original model and either do it as it should be done or the way I did for the Lysander. If you have the Lysander you will see it works as such but in most stock models these overlays are part of the actual aircraft meshes and not a separate mesh as I did.
Sorry to say I can help no further, the IL-2 bible I think has a section on overlays and there was something posted by an old member in the school about how it was done so maybe doing a search would yield better information than what I have provided.
Take care and be safe.
Wishing you all the very best, Pete.