The Mitsubishi Zero - Power Plant

The standard engine fitted to the Model 22 was the 1130hp Nakajima NK1F Sakae 21 fourteen-cylinder radial equipped with a two-speed supercharger. The official RNZAF report from the Engineer Officer at Hobsonville quotes this aircraft as being fitted with the Sakae 12 motor. This model, normally fitted to the A6M2 model, was lighter, shorter and less powerful than the Sakae 21. Fitting of a Sakae 12 to an A6M3 like NZ6000 would have posed weight distribution and center of gravity problems on the aircraft.
A visual inspection of the exterior of the engine currently installed failed to disclose any maker's plate, but a comparison of this engine with photographs of the Sakae 12 and 21 engines shows it to have the bulbous reduction gear housing around the front of the crankshaft characteristic of the Sakae 21. Thus we can take the Air Force report as incorrect, and accept that the correct motor is fitted to NZ6000.
The Sakae 21 was known, when under development, by the Naval designation NK1F:

          N - manufacturer (Nakajima) K = air cooled 
          1 = sequence number within given class of engines 
          F = version of the engine (i.e. 6th version)

When put into production, the engine was known by a name followed by a model number, in this case Sakae (Prosperity) 21.
The Japanese Army had a different designation system. and the Ministry of Munitions later devised a joint system for both Navy and Army engines. In this system, the Sakae 21 became known as the Ha-75 21:

          Ha = abbreviation for Hatsudoki (engine) 
          3 = air cooled, double row, 14-cylinder radial 
          5 = 130mm bore. 150mm stroke 
          21 = Model number for specific variant in Ha-35 series