Likely the easiest is the 2G 1200 aluminim swingarm...
From an appearance standpoint, the 1200 swingarm looks a little different, but not enough to really notice, unless your a Bandit guy.
If your trying to reduce the weight of our lovely little 600s, the basic frame is pretty heavy, exhaust is VERY heavy, forks, triple clamps and wheels are heavy, etc. Not an easy bike to get the weight down on, but then, it is pretty reliable little bike due to some of those components.