long ago vs lang syne

long ago is not a noun while lang syne is a noun.

Word	Noun	Adjective	Verb	Adverb
long ago	No	No	No	Yes
lang syne	Yes	No	No	Yes

As adverbs, long ago and lang syne are synonyms defined as:

Other synonyms of long ago include langsyne, long since.

long ago (noun)	lang syne (noun)
	past times remembered with nostalgia

long ago (adverb)	lang syne (adverb)
of the distant or comparatively distant past	of the distant or comparatively distant past