Why would ye be requiring frame diagrams, and what level of accuracy is required ?
If it needs (extensive) repairs, a good repairer should be building it back so that everything fits,
and merely confirm the dimensions are about right (?).
Stu Rogers was constructing a frame from the floor up, when I visited some time back
Ask him for advice, he likes a chat.
Tanks are about, I bought a replica from a maker in Kent some time back,
although it was in alloy and bolt-through. But he could equally have done it in steel.
Asking Stu Rogers or Paul Norman may well produce such a maker, they tend not to advertise.
It will be expensive though.
And 'original' ones are likely to be junk, or terribly expensive to get back into good shape.
Or just terribly expensive !