How come we do not have any for our flight sims?
Soon, very soon (though not limited to just a stock/straight F-51D). ; )
In 1947, with the formation of the USAF, the "Pursuit" designation was dropped, and all of the fighter aircraft in service at the time, like the Mustang and Thunderbolt, were redesignated with "F" for "Fighter" (where as during WWII the "F" designation stood for "Foto", and was used to identify camera-equipped aircraft - with the formation of the Air Force, this was changed to "R" for "Reconnaissance"). You'll also notice that the buzz codes changed at this same time, from PF to FF, and the stars and bars gained the red lines.
Quite a number of changes were made to Mustangs in service during and after this time, through active duty and ANG use (as well as foreign use) - this included different radio installations, different payloads, different stencils, new cockpit placards, a different propeller type, etc., that weren't seen on these aircraft during WWII. Other improvements were made to these aircraft after WWII, which saw different configurations of cooling gills added (on the sides of the radiators), an emergency coolant door release which hadn't come around until very late Mustang production, replacement and re-positioning of the guns/gun sight switch (and other items like that), and as Quicksand states, in 1953 a technical order was issued stating that the tail wheel units were to be locked in the down position (which of course, by that time, only would have affected Mustangs operating in the U.S.). The types and amount of changes differed from one example to the other, as the work was usually only accomplished when the aircraft was in for maintenance reasons. Also, although many Mustangs continued in service with their original interior green paint in the cockpits, others, when overhauled from 1950-onwards, were sometimes given all-over black paint applications in the cockpit as well (though not nearly as many F-51 cockpits were black, as some might be led to believe).
More details on this will be posted soon.