South Korea isn't just selling ammo to Ukraine. They are also selling lots of military hardware to arm Poland to the teeth. This includes tanks and artillery.
I am fairly sure that Russia will try to be restrained, at least initially, and it is likely that lower tier hardware like the MiG-35, Yak-130, and Kilo submarine will be sent. If South Korea then sends weapons into Ukraine, then this could be upgraded. Higher end weapons could be provided. Lada submarines, Su-75 when it is available, etc.
Licensed tank production could also be furnished to North Korea just like they setup T-90 tank production in India. Or aid could be furnished to help improve North Korea's own native tank program.