Good description here on how the remote booster works:
Remote Brake Servo
Moss U.K. has them for sale here - both Lockheed and after-market versions:
https://www.moss-europe.co.uk/remote...servo-kit.html
MBP offers a similar booster in the U.S., though the description states "Recommended for applications using disc brakes in the front and drum brakes in the rear."
https://www.mpbrakes.com/pc_product_...A40191940E1C58