Links to the source material, that is, the manhua that The Long Ballad is adapted from are given here. It should be available in all manga/manhua/manhwa sites, as per my knowledge.

https://mangadex.org/title/9510e82e-cfbe-4dda-823e-e3576420c732/song-of-the-long-march

https://mangakakalot.com/read-ii1sb158504884465

 Arbaz Akhtar:

THIS WAS SO GOOD TO BE LEFT UNCOMPLETED DAMNIT IT S BEEN 6 YEARS ALREADY NOW

Yeah, I know right? It was too good to be on eternal hiatus. If only the author started it again :')

 Arbaz Akhtar:

Truly a shame the manhua has different plot so it's interesting that we have two versions more fun but unfortunately ?. I don't think we will ever see that being completed 

I agree that getting two versions in manhua & drama are individual pleasures, as much as I would really love to see the last arc get adapted.

 Arbaz Akhtar:

I don't think it can get adapted last part of drama changed nd adapted most of the chars with different plot maybe they can still adapt a s2 with some changed key points nd i wish they could explore Ashina sun khitan origin in s2 nd more as he becomes their leader nd expands them later this all wasn't shown in manhua yet by the time it got cancelled fully 

Maybe the author would show how the characters become later, but well it got cancelled. The peace alliance of Tang with the grassland tribes was something she was getting at too, in the manhua but I would really love to see Ashina Sun & Li Chang Ge as the heir to Khitan & Uyghur tribes respectively :')
Maybe the antagonistic Khitan, Uyghur & Ashina tribes could come under peace alliance by their bond.