The Maserati MC12 have been completely developped by Ferrari. It has F140 (project codename of Ferrari ENZO) carbon chassis, engine and suspensions, only some lay-out items have been adjusted to be adapted at new longer body. It has brake steel discs indeed, not admitted in GT1. I posted pictures of the mulotipo based on the Enzo and used to develop racing GT1 version.
This was a choice of Luca Cordero di Montezemolo to relance Maserati name when Ferrari owned Maserati until 2006, today Maserati is joined to Alfa Romeo.