I work in the steel industry and we used hundreds of these valves in different sizes on an electroplating line I was involved in maintaining. There were two main designs, the "A" type (also known as the weir type) as here, and the "KB" type which was a "straight through" design.
The diaphragms come in different compounds depending on the application and what substance the valve would be in contact with (water, acids, alkali, oils etc) and the grade was identified by a letter on the small tab sticking out of the side of the diaphragm.
It might be worth checking prices for alternative grades of diaphragm (there are tables available online from various industrial pipe & valve suppliers). There may be a cheaper spare available; if I remember correctly a "Q" grade was suitable for water only but was much cheaper than the chemical resistant grades we had to use, since I doubt you will ever be filling the compartments with fuel again (although since the price of fuel has dropped in the current crisis, it might be worth stockpiling some).
Thanks for posting the restoration of your Militant's on here. I've followed them with great interest.