You can't commit this without consent from maintainer, this could possible fall under blanket but to avoid unnecessary friction I'd suggest that you submit it to bugzilla too.
https://docs.freebsd.org/en/books/porters-handbook/book/#porting-submitting

Given that this seems to be more or less abandoned upstream is this something we need add to the tree?

Looks reasonable but you need to file a bug report in bugzilla for Porter's Handbook and Committer's Guide to apply

Friendly reminder that maintainer timeout etc only applies to PRs ins bugzilla not Phab

Out of curiosity, would there be any benefit to add support / replace fftw3 with https://github.com/kfrlib/kfr ( math/kfr ) ?