USA might allow for Japan real army. And Japan is probably even more powerful economic power in East Asia.
America has always pushed for Japan to get a "real" army. The issue is that the Japanese people, for the most part, don't want one. WWII really discredited the idea of warfare with the Japanese public, and they figure, hey why bother when America is bound by treaty to defend us anyway? A unified Korea might encourage the Japanese to change their mind, but I doubt it.
The economy will hurt. As for the LDP, it will depend on the POD and the reasons for why the US/UN didn't get involved.Also this would be a major blow if anything to the Japanese economy. IOTL the Korean War was a huge boost to Japan, who had been struggling to recover from WWII. ITTL Japan will take much longer to recover and the LDP might have a harder time consolidating power.
And more Koreans suffer under Kim Dynasty.
Presumably there is no US intervention to stop this from happening.
Does it makes a difference if the US actually does try to stop them as IOTL but fails? For example the Pusan perimeter falls? Is this a more plausible POD than no attempt to intervene at all?
Note that the JSDF is a "for real" military and has been since the 1954 Self-Defense Forces Act.
Assuming the Kims are still in power. A unified Communist Korea and no perpetual stand-off against the south throws up plenty of butterflies. For one, Kim Il-sung was able to use the failure of the Korean War to purge a lot of his political rivals, such as Pak Hon-yong. Without the perpetual siege mentality Korea is unlikely to be as totalitarian as OTL, and would probably undergo destalinisation and a degree of liberalisation. You also wouldn't have the devastating effects of the Korean war on both sides of the border, or the subsequent militarisation of Korean society.