what are you up to? What's your timeframe? What are your goals?
Difficulties in terms of the bike in Korea are nearly the same as in Japan. ...Some kind of conventions which likely won't be enforced. Additionaly, it's not allowed to use bikes on any highway, which makes the airport a very interesting place to leave since no other roads are available.
Japan was surprisingly easy with my Carnet. ..Right, you don't have one..
I just loved Japan. What a great country which is definatly worth visiting.
Some friends of mine took another route which you may like aswell:
They flew the bikes over from Ulaan Baator to Bangkok (I can find out which company they used), crossed the chinese border, bought some 125cc bikes for 400US$ and rode all the way to the other side without any kind of escort or hassle.
Maybe this could be an alternative.