Yes Nava is part of Italian companies that work with the ACAR company. I met the owner of ACAR few years ago, he speaks French and English, he knows personally most of famous Italian company owners.
www.acarmilano.com
Where are listed biturbo gaskets on the Nava site ? I could only find classic Maserati.
There is also CPS for pistons near Milano. However they are only good to copy pistons. Last time I ordered a set of custom pistons they did a crappy work so I lost 800e. They don't speak English, are unable to design anything, they just machine. But they are very cheap: 200e per piston. Regular price is about 300-350e
Beware of the cheap biturbo head gaskets sold everywhere on the web: the black ones with a red sealing.
Below is a comparison of such a cheap gasket that made 20-30k km and less than 10 years, and the true Maserati (gold color) gasket that is from 1989 and made 120k km. The cheap one did not stand and is falling apart. Both come from the same 2.8 18v engine that was on my daily driver, I pulled out the engine a month ago.
I was thinking of ordering a modified version of head gaskets to improve sealing around studs and the oil passage. But I still have 3 sets of 2.8 real gaskets so not sure I'll do this soon. However if someone wants to start this work I would take part of the order. The gaskets are the same for 2.8 and 2.5.