I have a Tanaka M700 AICS, which is very similar to the KJW.
Specifically, the way that the hopup "hole" is cut into the KJW inner barrel is different from the Tanaka. The inner barrels are not interchangeable because of this. The M700 uses and AEG-style barrels and a VSR-style hopup sleeve; the Tanaka has it's own proprietary hopup rubber and inner barrel-hopup cut (a hole instead of a notch found on AEG barrels).
FWIW, the KJW barrel has in inner diameter of 6.10, so a new inner barrel is almost mandatory. I would suggest a DB Customs 6.01 tightbore and a high-quality (Prometheus or other) VSR-style hopup bucking.
Also, run it on duster, or you won't have any friends who will play with you.
Some links to check out:
http://www.airsoftforum.com/board/Ti...0-t101140.html
http://www.airsoftretreat.com/forums...?topic=10770.0