Some of the earlier posts talked about a radial engine in an AD Skyraider or similar. The good news about that route is that once you dis-assembled a R-3350 in a museum and made tracings from all the parts, you would be very close to having what you need to build a fully developed engine. Down side is that a current source of 130 grade aviation gasoline is almost more rare than the engine.
But what if instead of a spark ignition / gasoline radial, the mythical 'we' decided to go for a compression ignition / diesel radial? A diesel could run on NATO standard JP-8 fuel eliminating one (big) stumbling block, not to mention improving the odds of survivablity after battle damage.
My vision is something like a Bristol Centaurus converted to two stroke diesel operation. I am still conflicted on whether it should be a uni-flow cylinder with sleeve valves, or slightly larger but simplier piston ported loop charged cylinders, but both could wind up in about the same horsepower range.
As far as mechanics to service such a beast, we are not starting from zero. Many young men before getting to the military will have worked on or studied the engine in one of those Japanese 'tuner' cars. There is far more commonality between those engines and a military sized radial (mostly a difference in size) than a gas turbine.
While I am expounding, I might as well go whole hog. I feel that the Hawker Typhoon (see wiki for pictures of the Centaurus powered Tornado prototype) or the basic P-47 airframe would be better fit with a ground attack airplane than the P-51 / Piper PA-48 airframe.
I think that 'pucker factor' for those on the receiving end of a Typhoon / Tornado with 4 x 20mm cannon and a brace of rocket projectiles or bombs would be just as high in 2013 or 2020 as it was in 1944 hedgerow county. Maybe more so. Even as diluted and as thread bare as the German army was by 1944, we are still talking about soldiers trained in German tradition, not the rable that play as soldiers in most of the current world hot spots.
That is all I think i know.
