Surprisingly, nagahydras (and regular ol' hydras) lack all-around vison. Instead, they get "a +2 racial bonus on Listen and Spot checks, thanks to its multiple heads."
For the ability scores, like so?
Abilities: A master naga's Strength, Dexterity, and Constitution are all adjusted according to the Changes to Statistics by Size Table as detailed in the Monster Manual. (+8 Str, -2 Dex, +4 Con for a Large base naga, and +16Str, -4 Dex, +8 Con for a Medium base naga). A master naga's Intelligence, Wisdom, and Charisma each increase by +6. Note that a master naga gains ability score increases as usual for its increased Hit Dice. Master nagas often put these ability score increases into Dexterity, to offset the size penalty.
Alternately, since all official nagas are Large or larger, we could give them a +2 racial bonus to Dex to offset the usual size adjustment.