Fyi, I have a similar project, but it is on a 'backburner'. For my 3AJ, I have collected a number of XTZ750 parts, but not measured them up, etc.... I have them stored in my garage - a set of forks, triple trees, and a hub to try.
Being a twin disc, the forks are further apart with 43mm stanchions. So the triple tree is an important part to get - I have a XTZ750 triple-tree but not checked the stem length to be the same. Fyi, the headrace bearings are the same as the 3AJ uses.
The stanchions of a XTZ750 are a similar length to the 3AJ, but I think the XT660 ones are quite different - which is why I targeted the Super Tenere.
I have a hub & spindle, and I was thinking of retaining a WM1/1.60 XT600Z front rim size, rather than using the wider WM2/1.85 rim size of the XTZ750. Thinking it would then keep the same handling balance? I happened to pick up a Honda NOS WM1 rim which happened to be a matching DID rim - I am hoping to keep all the original 3AJ parts complete, as a fall-back.
The Super Tenere front brake calipers are a bit like gold dust. I have a spare pair of calipers (& master cylinder) for an early Hinkley Triumph Tiger, and I may be wrong, but I think I may be able to use these, (but I have not double checked!)?
That leaves the front brake discs still to obtain. However, I do have a pair of those plastic protectors!
I don't know if the above helps?
Drop me a PM/email if you think I can help more.
Fyi, I am unlikely to progress 'my project' for some time yet, but may be able to find my parts and measure, photo, etc...