The problem with bringing back any legacy system is supply. There are literally thousands of parts in something like an airplane. Even if you have all the technical specifications, the vast majority of those parts are no longer available and the companies that made them no longer exist. The tooling to make specialized parts has likely been long thrown away, destroyed or lost. (I have been involved in reverse engineering legacy systems and, mostly, they have to be redesigned and that is not cheap.)
Planes built today are designed for the add-on systems they will carry, which is much cheaper than trying to retrofit those sensors and weapons into a spot that was not designed for them. Also, piston planes require a type of gasoline that is forbidden by the EPA. (Not to mention a host of other issues that make piston engines more problematic than the far superior turbine engines that replaced them.)
Using the existing frame, use lighter material where possible, then build in the new technology. You have a proven airframe for a low cost fighter that can be mass produced. The two biggest problem would be has GD disposed of the manufacturing equipment and the engine. You do not want to use old engines. Everything on an aircraft has high time. Anything currently associated with an F-16 has been through depot level repair more than once. All I am advocating is the use of an outstanding airframe. Capable of 9Gs and proven to be reliable. As for the A-1 as of 2017 the USAF was already testing various existing turbo prop aircraft to fill the A-1 role. Fuel type is not an issue. As far as I know the Navy and Air Force still use JP5 and JP4 which fueled those older aircraft. But again I am not advocating for bringing back the A-4M as it was when it retired. That would be nearly impossible as MacDonald Douglas sold the manufacturing equip as scrap years ago. Just pointing out we do not need to reinvent the wheel to fund the military industrial complex.