We don't have synonyms for castrato yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are castrate, castrated and castrati.
(n) :
a male who has been castrated, especially a male whose testicles have been removed before puberty in order to retain his soprano or alto voice; a male soprano or alto voice produced by castration of the singer before puberty
(adj) :
castrated; especially castrated prepubescently; having, using, or containing the voice of a castrato (noun)
"nowadays, either women or countertenors take the castrato roles."
(n) :
a male who has been castrated, especially a male whose testicles have been removed before puberty in order to retain his soprano or alto voice; a male soprano or alto voice produced by castration of the singer before puberty
(adj) :
castrated; especially castrated prepubescently; having, using, or containing the voice of a castrato (noun)
"nowadays, either women or countertenors take the castrato roles."