It would be a fair bit less work than a new aircraft. Compared to the P-51B the outside shape would need some reshaping, in particular the nose, propeller, ventral scoop, rear windows and maybe a non-Malcolm hood. Also the guns, whether cannons or mgs. The cockpit would need modifying, as the layout was a bit different and possibly the canopy framing would need to be added. Same with the flight model, essentially the same aerodynamics but power, prop forces and weights are a bit different.