The title says it all - should a (e.g.) Sefardi say Amen to הַמֶּֽלֶךְ בִּכְבוֹדוֹ תָּמִיד יִמְלוֹךְ עָלֵֽינוּ when in a shul where they say this *beracha*, when he hears it from the Chazan? 

This is a beracha that Ashkenazim in Chutz Laaretz say as the final beracha Birchat Keriyat Shema of Maariv, before the Amidah and (some) Sefardim do not say it.

For example, in Yalkut Yosef Kitzur Shulchan Aruch 25:54, and OC 422:4, he writes that Sefardim shouldn't answer amen to Ashkenazi Birchat Hallel when it is half-Hallel, and shouldn't answer amen to Ashkenazim making a beracha on their Tefillin Shel Rosh (because Sefardim do not say these berachot themselves).

The principles behind not saying amen to a Beracha you do not say yourself - does that principle apply here?