Can an improvement to an existing implementation be publishable?
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.)