Laycock, Donald. Three Upper Sepik Phonologies. 1965. wals:3490
Glottolog lists this source as "Iwam unclear which one probably iwam1256".
/ɾ̃/ is described in the source as a velar nasal /ŋ/ with an "alveolar flap [n̆]" allophone that appears initially and intervocalically but not finally, but taking the word-final allophone as the underlying form is maybe a stretch.
Vowels are usually nasalized when adjacent to nasals and "frequently initially and finally".