The Gnomish Lexicon of the 1910s had G. maron or marn “ripe” (GL/56), clearly cognate to ᴱQ. marin(a) “ripe, mature” from the contemporaneous Qenya Lexicon based on the early root ᴱ√MARA¹ “ripe, yellow” (QL/59, 63). Early Noldorin word-lists of the 1920s had ᴱN. marn but without any translation (PE13/150).

Neo-Sindarin: We have no later word for “ripe” so I would retain ᴺS. marn “ripe” for purposes of Neo-Sindarin. The early root ᴱ√MARA¹ was a variant of ᴱ√MALA² “yellow”, and √MAL remains the basis of “yellow” words in Tolkien’s later writings. I would assume marn “ripe” is the result of some ancient L/R variation on the root √MAL; compare S. -rim vs. Q. -li “many”.