Only that the one in the National Motorcycle Museum in Birmingham is a replica, so it is surely not an impossible task to build something similar.
Unfortunately, because there were only 10 Brough-Fours manufactured by the factory information is pretty sparse - particularly technical information. I suggest your best bet would be to contact the Brough Club
What I can tell you is that the forks were made by Castle, were of the leading-link type, and, to the best of my knowledge, were copied from a Harley Davidson design - a design now being manufactured brand new for modern Harleys, no less!