In 1957, the last of the active duty F-51s were withdrawn from ANG service. This released many F-51s to the civilian market. David Lindsay, a newspaper publisher, formed Trans Florida Aviation with the intent of refurbishing the ex-military P-51s into well-equiped civilian business aircraft.

Lindsay purchased surplus P-51s (mostly P-51D) and began a restoration process. They would strip out all the military equipment, add a second seat behind the pilot, add extra fuel capacity (some models), update the avionics, install a tall tail similar to the NACA P-51s, plush out the interior to provide the most comfort possible and finish the job with a civilian paint scheme. Lindsay flew 44-13257, with the NACA tail, and was pleased how it handled. He got the FAA STC for the tail mod so the taller vertical stabilizer could be added to the civilian Cavaliers.

One of the P-51s main attributes was its great range. The first P-51 conversion, named Cavalier, was in 1958. Orders trickled in for the first few years. The models offered were all relative to the range of the aircraft. The model numbers (all prefixed by "Cavalier") were 750, 1200, 1500, 2000 and 2500. The longest range Cavalier, the Cavalier 2500, included 110 gallon wingtip fuel tanks. Remember, with the second seat, the fuselage fuel tank was removed and the main wing tanks would yield about 180 gallons usable.

Trans-Florida was renamed Cavalier Aircraft Corporation and purchased the rights of the P-51 design from North American Aviation. The first contract with the U.S. Government was to IRAN 36 F-51 from the Dominican Air Force (FAD). These were not Cavalier conversions but were P-51s that were repaired and upgraded. Then in 1967, the USAF contracted with Cavalier to produce the F-51D for export to South America under the Military Assistance Program (MAP). These aircraft were given new serial numbers starting with the first, 67-14862 and named "Mustang II". In 1967 a total of 9 were built. These aircraft went to El Salvador.

Changes to the Mustang II were made for increased loads. The wing was strengthened to carry a total of 4,000 lbs of ordance and additional weapon hard points were installed, up to six under each wing. A rear seat was installed in these models, for observers. A new Merlin V-1650-724 was installed and these Cavaliers also received the taller tail modification. In 1968, two of the new Mustang IIs went to the U.S. Army as chase planes for the Cheyene (AH-54) helicopter program. These were serialed 68-15795 and 68-15796.

More orders placed in 1972 for 6 aircraft under MAP for export to Indonesia. The Mustang IIs did not have wingtip fuel tanks.

With new ideas of how to keep the P-51 Mustang alive and in service, David Lindsay wanted to try replacing the long-standing workhorse Merlin V-12 with a turboprop. Lindsay perferred the Lycoming T-55 but had difficulties obtaining a copy. They were able to get a Rolls Royce Dart 510 Turboprop and installed it in civilian P-51 N6167U. This mod was not funded by the USAF, but by Cavalier.

The new modification was called the Turbo Mustang III. Cavalier tried to get the USAF and other air forces interested in the project but no sales were made. Later, Cavalier sold the project to Piper and it later became the PA-48 Enforcer. The Enforcer had little in common with the original P-51.

The USAF, under pressure from Congress, did order two prototype PA-48 from Piper. No other orders were place and the project died. The two PA-48 Enforcers do exist today at USAF Museums. Many Cavalier Mustangs are still airworthy today.