Anyone who follows North Korea can get an idea of the country's industrial base. And not only that. In the DPRK, there are a huge number of facilities and research institutes focused on fields such as aerodynamics, thermodynamics, material engineering (including large-scale production of various...
How are you going to put an engine weighing a ton more(!) than a pair of J85s into something you call a Kowsar NG?
It will be a completely new plane with a new fuselage.