I would think this 'TR6' one on the same site looks a better tentative match for upgrading a single-circuit disk-braked, servo-ed HB to dual-circuit:
https://mossmotors.com/brake-master-cyl ... ftermarketI would of course need to know the diameter and the depth of the boss on the end, and the distance between the mounting-hole hole-centres, to either rule that or any given master cylinder out, or consider it a 'hmm, possible'. Data like this should be available up-front with their products, but no doubt a query should elicit some useful info. It doesn't mention the bore at all, but it's reasonable to assume it should be adequate. Two circuits means twice the input effort required and the bore I expect would be on the largish side as the effective piston travel is halved.
Taking into consideration the masking and force multiplying effect of the servo in-between, by pretending its not there:
A smaller cylinder bore will give a lighter pedal, but it would have to travel further to build the same pressure as a larger bore one would, and might run out of travel before effective braking is applied, or at the other extreme be so light as to easily lock-up the wheels; a bigger bore will give a harder pedal, pushing back more, a wooden-feel if unassisted, and would take up slack and build pressure and thus activate sooner.
Trial and error, and finding what other people are doing/thinking and learning new stuff is the only way to find what fits and works well and will be sustainable into the near-future. It's no good someone saying they got their's sorted with something that is equally as rare and exotic as the original-equipment kit is becoming or will one day become. Aftermarket options - and with that increased safety margins, compatibility with other classic models assuring longevity and availability and continuing spares support - definitely looks the most promising route in the medium term when keeping the OE stuff serviceable could be grimmer still. I've no desire to pioneer in this area, at the moment brakes are immaterial as another bout of welding/bodywork/paint is in progress and last I recall the brakes were good to excellent, and spares are readily to hand, but take an interest in the subject with an eye to the future.