The 748RS carbon tank has two chambers that recycle water and just catch fuel. There are five hose fittings, 1 for water in/out, 1 for water vent (like stock), 1 for fuel drain from cap, 1 for fuel vent from cap and 1 to drain the fuel from vent tank which has a drain plug. The 998RS tank (also two chambers) is part of the front airbox. Sten, you should be able to use the 748RS one on your 998R.

Hope this helps.