You are retro-fitting power brakes? Mangusta's had non-powered breaks. Much easier to locate the Girling (non-power) booster. I don't have the info handy but I recall the unit was easy to find and not terribly expensive. Also, some of the brake lines are soft rubber, which I would imagine must to be converted to hardline if you go with power brakes. I would be a little surprised if that booster fit under the hood. I'll try to measure for you next time I'm in the garage.
Good luck!