North Korea Naval Bases and Fleet HQs. Mini-submarines and Sinpo-Golf "SSBKs" might operate out of Sagin Ni (under West Sea Fleet Command) and/or Sinpo - Mayang Do and Chaho Submarine Bases (under East Sea Fleet Command).The likely North Korean nuclear missile launch points (including SLBMs) would have been defined by Russian, Chinese and Western military intelligence for years, with constant sensor surveillance and cueing of first and/or second strike forces. Golf SSBKs could also be seen as Sinpo class depending on the extent of modernisation-component replacement. SSBK means conventional diesel electric propelled submarine capable of firing large nuclear tipped ballistic missiles. (Map courtesy US Defense Intelligence Agency (DIA))
I estimate the NKs renovated or are renovating around 3 (military "Rule of Thirds") of their very old ex-Soviet Golf SSKs up to SLBM capability. In 1993, ten Golfs were sold to North Korea for scrap. According to some sources, the North Koreans are attempting to get these boats back into service.
It would be unnecessary to reverse engineer whole Golfs but mainly integrate some modern features (especially electronics and engines) into the three Sinpo-Golf SSBKs.
These Sinpo-Golf SSBKs are probably very noisy due to their obsolete shapes and reverberations of incompatible equipment. This deficiency is made worse for NK by many Western, Chinese and Russian sensors (including sonar) being cued against the Golfs and against other potential NK nuclear missile launch points.
Due to easy detection of the Sinpo-Golfs SSBKs NK would likely pursue a stategy of keeping them in NK (missile and aircraft) protected waters - much as China has done with its noisy Type 094 Jin SSBNs.