Japan is not likely to develop the X-2 into a fully operational stealth aircraft—the cost is just not worth it for a limited production run. Instead, the country is likely setting itself up to participate in a future co-development effort with an international partner. The most likely partner would be the United States, which is gearing up to develop the F-X and F/A-XX follow-ons to the F-22 Raptor and F/A-18E/F Super Hornet.
The Japanese have long coveted the Lockheed Martin F-22 Raptor, but U.S. law prevented the export of the stealthy air superiority fighter.
The US sixth-generation fighters are expected to use advanced engines such as Adaptive Versatile Engine Technology to allow longer ranges and higher performance. Risk reduction began in 2012 so that engine development can start around 2020. An engine is to be ready when fighters are introduced by the Navy in 2028 and the Air Force in 2032.
The ADaptive Versatile ENgine Technology (or ADVENT) program is an aircraft engine development program run by the United States Air Force with the goal of developing an efficient variable cycle engine for next generation military aircraft in the 20,000 lbf (89 kN) thrust class.
SOURCE - National Interest, Wikipedia