I would certainly look for Revell to have the Liberator parts.
As for the P-51, those would be really easy to make, and figure they are down below anyways where no one will pay much attention. Usually, I do replace landing gear doors with thin styrene, or if curved, beer can material.
I'll bet if you enarged the instruction sheet or even the box art, you couls easily get a serviceable pattern, or do a pencil rubbing of the hole in the wings.
Get into scratchbuilding, its fun. Our old long gone friend Hans Von Hammer taught me that if you try to make something yourself, basically no one can reasonable say it's wrong.