If you wanted to make M14s again, you can collect up all the drawings, and the specifications and you would have all of the information necessary to make M14s just like the old ones. I think somebody has basically done that.Great discussion, and I really love to think: just take all the original drawings and tool up and make new ones .... but having managed engineering projects through my career, I sadly have to agree with the logistical truths that you cannot keep an ageing fleet going forever.
There ought to be a way to make an A-10v2 using original design concepts, though the challenge here will not be engineering, but political, like the Bradley Fighting Vehicle, where politics and differing opinions will allow scope creep to poison the design, so what comes out is something that is a compromise based on negotiation, not performance.
Unfortunately, aircraft aren't like guns.
1) Much of the design isn't actually on paper, it's in the tooling. Tooling to form the skin, frames, valve bodies, duct work, etc. And when that is destroyed or repurposed to other projects, it is gone forever.
2) Much of the technology used in the A-10 is obsolete and no longer available. The computers used to control various aircraft functions are 1970 vintage technology, and the chips are so obsolete you cannot make them anymore. And these are the core aircraft functions, not the easily modified mission avionics. To design and build a new modern computer to do the same jobs, would require a long and expensive development cycle, and a full validation test. Simply put, it would cost as much in time, money and effort as it did back in 1972.
3) "There ought to be a way to make an A-10v2 using original design concepts . . . " They did. they also threw in a few other additions, it known as the F-35. And your right, the problem is not engineering, it is political in getting people to understand that it really can do the core mission of the A-10, and other strike missions as well.
The only major difference is the F-35 doesn't have two tons of machine gun in the nose.