Can an improvement to an existing implementation be publishable? My master thesis is an improvement of a code. I gave this code about one year and half ago from the author of the one paper.

After working on his code, I improve the result. but my implementation is not a lot. My added part to his code is less than 300 lines.

The base paper was published on 2013 in the journal with impact factor 2 .

My question is "improvements works" have the ability to send to a journal? Or when you can send your work to journal which you should develop the whole things.

By the way the issue which I am working on is one of the new standard.
For purposes of publishability, it doesn't really matter whether your implementation was built from scratch or from an existing codebase, or how many new lines of code it required.

What matters is whether your results contribute some new knowledge to the field....

... which we can't evaluate for you here. You need to read the literature and the previous work, and identify what you're contributing that's new. (It's not measured in lines of code.)