By the time you got done modifying the U.S. WWII mask, you might as well have made your own, which would be more accurate and much better looking. Everything about the lightweight gas mask in the set is wrong for modification, from the bag, to the lenght of tubing, to the very shape of the mask itself. You would need to carve new eypieces, fabricate new fittings, make a new bag, create new shorter tubing, create many many folds in the facemask, and also pruduce the flatspot where the nose clip is. You would also need to add material to the sides where the mask expands outwards, and also remove some from the chin and forhead.
The SBR is VERY simple in construction. You would need to make a putty mask with eyepieces. Then, using small tubing and some more putty, make the hose fittings and the flutter valve. The bag would probably be the hardest to make, but still only a square bag with a single flap. The tie around string would be difficult too. The hose could be anything with the right shape to it, though it may be hard to find.
Just my $.02
