Is it ethical to publish a paper about porting an existing software package to another programming language? In short:

I have ported a package in another programming environment, and I want to write a paper of my ported package. Would it be considered cheating?

In details:

I have implemented a package in Python, that is modeled after an R package, which means my package has the design and APIs similar to original's.

The authors of original R package has published an official paper about the package on Journal of Statistical Software. And now I want to write a paper about my python package.

The point is, since the design and APIs are similar, the structure and content of the my paper would be similar to that of the original package's paper. For example, in the original paper, the authors explained what Corpus class is, and in my paper, I would explain that my python-version Corpus has similar attributes.

The biggest differences are brought by the environments and dependence, and some language-specific details.

So, would it be considered cheating if I write the paper of ported software? Of course, the relative sections of my paper would include the comparisons between two packages and I would cite the original paper as appropriate as I can.
Your paper must clearly delineate what is your original contribution and what is the same as, or based on, the previous package. So if you write about your Corpus class, for example, you must say that it is modeled after the R package's Corpus class. Otherwise it is "cheating," as you put it.

(It's also not clear to me that a straightforward port of an existing package would be considered a novel research contribution by most venues for publication. But that's an entirely different question.)