If you bought that dinghy specifically for rowing, then you certainly bought the wrong boat! RIBs are absolute pigs to row! You might want to consider a small Whitehall style row boat instead. They are a dream to row, and you can make good headway even against wind and waves!
Of course inflatables definitely have their strong points as tenders. We have an 8' Aquapro, and we use a Yamaha 4hp 2stroke on it. The engine weighs under 50lb and has a full F-N-R transmission. If you want the lightest engine possible, try to find an old Tohatsu/Nissan/Mercury (They are all identical except for branding) 2.5hp 2 stroke. They are dead simple, reliable and ridiculously light. They have F-N only, and you spin the engine 180deg for reverse.