Peter,
The Sunbeam Tiger also had a remote unit and maybe the Sunbeam Alpine had it also. Probably the same Girling used in the Lotus, AH, etc. Girling supplied 5" and 7" sizes depending on the assistance required.
The original Girling units are no longer made, but used ones maybe found at a swap meet or junk yard. Rebuild kits are available, but are incomplete. They do not include the bits for the vacuum piston. The unit gets a bit iffy when the vacuum piston is unreliable. Sometimes they work like unassisted and sometimes like power brakes. Life has its moments when stepping on the brakes can produce radically different results. Try to get one with a good vacuum section.
Lockheed makes a replacement unit. Not exactly the same dimensions but same concept. It is expensive.
Robin's suggestion makes a lot of sense. There is plenty of room else where in the engine compartment and you could go for a split braking system at the same time.
You are probably very lucky in to the cattle lorry versus Ace incident.
Peter