If the old booster and slave mater cylinder is serviceable then I would stick to originality and service it yourself with seal kit or Omicron can do this for you and test it, as they have rig. As modern dual circuit solutions aren’t the nicest of things and can be quite bulky.
Regarding brake pistons, these bolt onto the claw/caliper. I have original Dunlops which are very good, although if yours are too far gone, you can get them relined or go with aftermarket kits. There are plenty of aftermarket products as they are the same that Jaguar used, there are even 4 pot upgrades. Dave Gee is the one to speak to as I think used
http://www.fossewayperformance.co.uk/lancia-flaminia-brake-upgrades/ on his sport.