To be sure, the more common modern English version is czarina or tsarina, but that appears to flow from a German ending being engrafted on the Russian word; “-itsa” is the feminine suffix in Russian. If you really care about respect for the great Russian people and their language, that’s what you’ll use, or else you’re a colonialist oppressor. (Actually, the Germans were indeed colonialist oppressors of the Russians for a while, though then the Russians took over oppressing some of the Germans.)
Of course, if you what you really care about is English, “czar” would be just fine. And ultimately all this “czar” stuff is a cultural appropriation from the Italians, so there’s that.