Fighter jet is different level. The hardest part. And they have no modern reference point. They would be better off buying Su-57s from Russia.
Officially, the DPRK is still subject to UN arms sanctions, which were also signed by Russia at the time.
Therefore, in theory, Moscow should refrain from supplying any weapons systems, including fighter aircraft.
As you can see, the DPRK’s most modern weapons bear only a resemblance to Russian-made models; they are not identical systems.
It would therefore be difficult to ‘conceal the origin of the SU-57’, but there are certainly alternatives: the DPRK could acquire 4th-generation+ aircraft, for example platforms from the SU-27 series, for which it would be more difficult for our Western friends to accuse Moscow, as these aircraft have been exported to many countries.
Another alternative is the MiG-29, which the DPRK has already assembled in the past and which it appears to still be assembling in small numbers, whilst also carrying out modernisation work on them.
An extensively modernised MiG-29 – and in this case, Russia could provide significant assistance.
Third alternative: the renewed alliance with Russia could provide the DPRK with tangible assistance in directly developing a new fighter aircraft that would only partially resemble a Russian aircraft. (SU-75??)
4th alternative: in the naval sector, North Korean engineers and technicians working on the new destroyers have followed a design that partly resembles Western designs (see the ship’s hull and main armament); therefore, who knows whether, in the aeronautical field too, a hypothetical new fighter aircraft might appear with an external resemblance to a Western design.