If the original is BSD licensed, you can (almost) do whatever you want.
As a courteous person, you will cite the original project as the basis for your version.
As a practical person, you want to be able to track upstream changes (and even better, have third parties help out doing so), add/reference any translation tools used (e.g. I'm aware of FORTRAN to C translators, TeX uses a translator for a (subset/dialect) or Pascal to C, there must be others), or state your hand-translation strategy. Clearly mark where you deviate from upstream, to guide later refreshes. Note that when I mention "third party", more often than not that will be you in a few months time, completely oblivious to what you were doing today...
Take a step back, and carefully consider if it isn't better for everybody involved to integrate your changes upstream (if they don't want it, perhaps they have their very good reason to reject them...), or keep your own branch with minimal changes (or some sort of plugin/addon?). Perhaps the best solution is adding a shim that allows using upstream in your language?