There were two engine types on the early version, not until the RR was strapped on did this become the rocket it is today (and during WW11).
"The Mustang was originally designed to use the Allison V-1710 engine, which, in its earlier variants, had limited high-altitude performance. It was first flown operationally by the RAF as a tactical-reconnaissance aircraft and fighter-bomber (Mustang Mk I). The addition of the Rolls-Royce Merlin to the P-51B/C model transformed the Mustang's performance at altitudes above 15,000 ft, allowing the aircraft to compete with the Luftwaffe's fighters.[nb 1] The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the Rolls-Royce Merlin 66 two-stage two-speed supercharged engine,"
Thanks for all the input ;-)