Regarding the hidden trusswork issue, why not just omit sheathing the trusses? Though the sheathing undoubtedly makes the 'walls' stiffer, the rebuilt pics show those heavy truss elements apparently very able to do their job without help.
Is it mandatory that EVERY original bridge feature be kept? It's going to be modified anyway to suit WW&F use.
Truss design and apps have always fascinated me, a brilliant way to safely span areas using just-enough elements.