To my knowledge all of the equipment that was on Kauai stayed on Kauai until it was scrapped. It was much more difficult to get to Kauai than to any of the other islands back in the days when steam was being run. Even today, it is much less developed than Oahu, Maui, or the Big Island. The reason that 4 locomotives did survive is due to the Wilcox family that owned Grove Farms being historically minded. The engine house and trackage currently in use are about 2 miles apart and the intervening distance has been filled with houses so I don't think you'll ever see them connected again. Its more likely a new engine house will be built when money becomes available.
There's also the Kilohana Plantation railroad that was recently built to 3" gauge with imported non-steam equipment. It is located a couple of miles to the southwest. They have no connection to the Grove Farm operation...