Wrong forum I believe.
To answer the question the first thing necessary is to not have Kim Il-sung. This would possibly lead to less focus on the military and more money to spend on the economy, which can help tremendously through the 60s, 70s and 80s.
The next challenge then is to stop North Korea to 1) become democratised and 2) become unified with South Korea. For this North Korea may need to become quite authoritarian, cracking down dissidents and the like; also necessary is to reaffirm the alliance with China. South Korea may need to become more anti-PRC in this TTL.