If one wants a complete grammatical analysis, one should be prepared for the view that be equal to is a transitive multi-word verb, a single lexeme.
("A lexeme is a unit of lexical meaning, which exists regardless of any inflectional endings it may have or the number of words it may contain. Thus, fibrillate, rain cats and dogs, and come in are all lexemes, as are elephant, jog, cholesterol, happiness, put up with, face the music, and hundreds of thousands of other meaningful items in English. The headwords in a dictionary are all lexemes." (David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge University Press, 2003) )
The fact that is equal to etc can readily be substituted by equals etc strongly supports the multi-word single-lexeme analysis. It then becomes arguable whether it is helpful to try to analyse within the fixed expression (along the lines: is equal more closely bound to the 'verb' or the 'preposition'? if the 'adjective'-'preposition' binding is tight, is to better analysed as a particle?).