Hmm, let's actually pull the divergence back to 1945
If we're talking about having both the Koreas and letting them fight, when then I'm afraid urban fox is right, North Korea wins.
I was thinking more along the lines of, what if the peninsula is not partitioned and occupied after WWII. For example, what if instead of some paternalistic mangement by the US and USSR, creating zones, the powers just recognize the Korean People's Republic declared in the country right after V-J day, or they recognize the government in exile in Nanking. Who would win the power struggle if the US and USSR are not in the country in such a big way setting things up (like the North Korean Army, which, equipment, organization and training-wise, was such a clear Soviet set up). So, would indigenous reds with or without a low level of support (like what the ChiComs or Greek Communists got) win on their own, or would they lose to the declared postwar regime?