why not replace the Allison's with Merlin's in the P-38?
Lockheed themselves looked at such a conversion at one point. Nevertheless, a P-38 was sent to Rolls Royce at Hucknall for a trial installation of Merlin XX engines. The program was initiated by the Americans as the P-38 was proving somewhat troublesome in the high altitude role over Europe. RR made a number of test flights in the unmodified state, but before conversion started an order came from Washington for the aircraft to be returned to the 8th Air Force immediately. The supposition is that, Allison having seen their P-51 business go up in smoke, were somewhat upset to see their P-38 business possibly following a similar fate, and applied the necessary political pressure.