Is it okay for a Chabad Chassid to wear Nusach Arizal tefillin that are not Chabad tefillin?
Some in Chabad wear tefilin in Alter Rebbe ktav, others wear Ktav Ari. But there are multiple issues in tefilin beyond the ktav (script). For instance, as I documented here, the spacing between paragraphs is a difference some find significant.
Also Chabad has a chumra/hiddur to use larger batim (40mm x 40mm at the top, e.g., see bottom of here, instead of the standard 35x35).
For all these reasons one would need (1) to understand the details of the specific pair of tefilin under discussion and (2) to CYLCR (check with your local Chabad rav).