it is easy to temp import bike or vehicle into South Korea.
BUT . . . . . .
depending on your driving licence country of issue (and/or passport country of issue) you may not ( MAY NOT ) be able to ride or drive there.
for example in 2007 we went through with a friend, we (british passports, british licence) had no issues, but the friend (german passport, german licence) was not permitted to ride there.
This all boils down to an agreement between countries to mutually allow other countrys drivers to drive/ride in the particular country, sort of an agreed recognition of any countrys verification to drive ie driving licence procedure.
Now, the original agreement made soon after the end of WW2 did not invite Germany (and maybe others) to the table to sign the agreement.
The superseding agreement made during the 60's then didn't include Korea (because of the whole north v south dispute going on at the time).
Consequently this has never been corrected and so to this day I think you'll find that a German issued driving licence will not be recognised.
There may of course be some other countries that fall between the cracks, I suggest contact South Korea Customs and Immigration to confirm.
Import company - we used Wendy Choi at aerokorea 'A'T' unitel.co.kr she is excellent, very friendly, very helpful (tell her phil sent you)
Ferry for us cost $500 for the vehicle (4x4) and $250 per person.
Time - overnight crossing, about 7 hrs if i remember rightly.
South Korea is fantastic country to explore, visit and drive. We stayed 7 weeks.