the only major difference is a crown race size. iso specs (what 90% + of headsets, including chris king, are manufactured to meet today) are 26.4mm races. jis specs are 27.0mm. chris king says that they'll make crown races ("baseplates" in their lingo) that are up to .2mm +/- their iso standard size. unfortunately you're looking at a .6mm difference.
option 1: ask ck about making a custom headset race for you to fit 27mm jis specs.
option 2: get a jis headset from velo-orange.com
option 3: take your alien fork to a very well equipped lbs, or framebuilder, and ask them to file and then face down the headset race to 26.4mm iso size. you can use a ck then no problem, but you'll most likely void the warrantee, if any, on your frameset.