Their own agricultural product? If needs must they can always start planting weeds/heroin and ship into Russia.
Mao starved 15% of the population to death with his Great Leap Forward and Cultural Revolution, Northern Vietnamese lived under terrible privation following the lane reform mandated by China after 1954, and went on to fight another 20 years war for reunification. You are severely underestimating the resilience of East Asia Communist regimes.
North Korea is nothing like those places, it's a complete mess. They don't have a functioning state, their government basically doesn't know how to govern. They don't build anything. They don't feed their own people. They don't tax anyone. Without foreign support millions more would've already starved.
To maintain this regime they would need to instantly go from being completely dependent on another nation, to becoming a revenue generating and functioning state.
Why don't they just start planting stuff now?