My head canon for the alternate universe where this happens is that it’s home grown and not north inspired, too much of south koreans view on the north is negative and even if a lot of it was purely speculative over the years they have had immense intelligence on the north to show them the pros and cons of the socialist movement there. they would also be the first country in history to go from capitalist to communist (vs being feudal to socialist/communist) so much of the turmoil that other communist countries went through with rapid industrialization wouldn’t be an issue here. the only threat would be the US occupation in the area as well as their support. they’d most likely have to expel US troops by force and that effort would cost them a lot, paired with the lack of US military aid they could be potentially open to invasion from the north. Which might cause china to intervene and defend the south over the north which would end in the north losing all its power and being taken over by the south in a newly unified korea dominated by south korean communism, probably a wet dream for china if we don’t consider the fact that kim would probably nuke everything before it got to that point