Hi Ian
I built a Maisie in the 1970s and after many years unused it was recently subjected to a hydraulic test by my club boiler inspectors. It passed without a problem.
LBSC produced 2 versions of the boiler, one being a bog standard design for the period and an advanced version with extended firebox and thermic syphons. I built the standard boiler and cannot see any reason for building the advanced one. My model and every other one I’ve ever seen running has produced more than enough steam with the standard boiler.
I still have a full set of drawings but unfortunately 50 years of garage storage has not been kind so they’re in very poor condition.
I would suspect that the design would be considered unsafe for new builds today, mainly due to the absence of bushes for many boiler fittings. I know of at least one model which had a failure at the smokebox steam pipe joint due to the lack of a bronze bush. Bushes of course could be easily added at the build stage so you need to discuss with your boiler tester before starting construction as there may be other features of the design (eg. girder stays on the firebox crown) which are considered poor practice by current standards.