Airplanes that are in the warranty period of time will have the parts sent free to the airline and compensation for the labor associated with the modifications. Airplanes outside of warranty are responsible for paying for it on their own.
The question I am not answering is what constitutes if an airplane is in warranty. It depends on the airplane, operator and type of repair. For economic improvements, warranty usually does not cover them except within a certain window. For defective designs covering safety, it typically does. Warranty almost never applies after the airplane has been transferred from its first airline to a second airline (unless it is owned by a leasing company and they have different rules).
If you have never designed an airplane part before, let the real designers do the work!