Hate to be the bearer of true news......but the Mangusta very much DID come with power brakes! Likely every last one of them did!
Booster is in the interior of the car, behind the instrument panel! So much fun to get at..........!
If this was a Benditalia (Bendix Italy????) piece, it could share parts with a similar Bendix boosters we see here?
Try a local booster rebuilder......but I presume you already have......they are rare fish to find these days! There is one left here in San Jose........
POWER BRAKE EXCHANGE - 2870 Aiello Dr, San Jose, CA
Cheers!
Steve