As the master cylinder is an oddball it might be worth looking for a modern replacement that is the same diameter. If there is room for the servo right at the master cylinder mount, look for a unit that can be mounted right in there. Some small cars have small diameter servos, but small cylinders, you might need to adapt a bigger cylinder to a smaller servo.
Having written all that, what may be the best scenario would be to rebuild the master, or replace with either something modern but the same diameter, or something old but still available like early ford or mustang. Then plumb in an add-on remote servo. These used to be available as a kit (Girling powerstop) and were used in lots of cars in the 60's and 70's. It can be mounted virtually anywhere.
Best of both worlds, keep everything standard, power brakes for the missus. The servo could be removed by a subsequent owner down the line if not wanted.
Mart.
Edit: I had a look (remote servo) on ebay and there are plenty of new ones for less than £100. Some say 1.90:1 ratio, some say 1.65:1, I'd go for the lesser ratio.