'Focii' is wrong for two reasons:
English treats proper nouns as English words when a plural is required. That's why the American gangster series is The Sopranos and not The Soprani.
The Latin plural of focus is foci. It is used in English, notably for the two point that define an ellipse.
At least focus is a Latin word, unlike 'Lexus'.
Incidentally, if all Latin-derived car names were to use Latin plurals, Volvos would have to be Volvamus.
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