It is a single file c source game that you can run from the command line and I would like to rewrite it in elisp.
When you translate a program into another language, that is a kind of
modification. The translation is covered by the copyright on the
original program. This applies to machine-translation and also to
hand-translation.
If the original program carries a free license, that license gives
permission to translate it. How you can use and license the
translated program is determined by that license. If it is the GNU
GPL, the translated program must be covered by the same versions of
the GNU GPL.
If, however, what you mean by "rewrite" is to write a new program from
scratch that does the same job, without translating the code itself,
that is not a modification of the original program.