Jamie, I'm not in a position to be of great help, just now owning and in the shop for adjusting my very first Exhibition in 40 years collecting... A bit late, isn't it?
But two things i can say.
About mica, I would suggest you to take the mica of the bad soundbox and test it. Holding the mica between your index and thumb fingers, vertically, just ½" over a hard table, release it so it drops that small height over the table, bounces up and there you catch it again, do it several times, and listen to the sound. A Cristal clear klinnk! must be heard, and the mica is good. If the sound is a thump, then the mica has an issue (delamination?). The higher the tone, the better the mica is. If you happen to have several examples, compare them visually and audibly through that Kling sound. You will see the differences with a bit of careful examination.
Also examine the central hole for signs of cracking or delamination. Although the central hole is not good, of the rest of the mica is pretty solid and tight you will hear the klink sound as well, but a bad center will make it work bad.
The mica must be clear transparent all through, without strange reflections. Mica delaminates in very thin transparent leafs. You must not see any of that all across it. Ideally the cut at the edge must be neat as well cut brown glass all along, same colour as the mica, but darker. Whitening is a signal of border delamination. But if you have this defect, the sound would not klink cleanly, most probably.
About the exhibition springs... I've found they are pretty hard, so this needlebar is not very compliant. But no problem, as this seems to be the original design. What is true is this: with such hard springs, the needlebar must be very carefully adjusted so it does not press the diaphragm at either side when relaxed. The needlebar must be adjusted before mounting the central screw. First, the two fulcrum screws must be pressed slightly, so there's no loose sensation when you move a bit the needlebar with your fingers, just like the record groove vibration would do. But the movement must be same amount as if a record groove were moving the needle. If this gives a loose sensation, the soundbox will surely buzz. But they must not be either too hardly pressed, for this would limit the compliance too much.
I've also noticed that sometimes it's a bit tricky to make the fulcrum rest on its due location: with both knife points in their grooves on the fulcrum plate. The first tweakings with the screws can misadjust that.
Adjustment as usual: the needlebar foot must touch the diaphragm very slightly. Then the central screw can be fixed.
And once assembled, it must be tested and tuned somehow. I've learned in this forum all these things, and studying the exhibition closely, all that has its own logic. When testing the soundbox, a certain sweet spot of tuning must be found by tweaking a small bit the spring screws. Move one at a time, and notice the sound changes. Movements must be very small, and the sweet spot could be found in either direction. If one tweaking doesn't improve the sound, then return to the original adjustment and try the other screw.
That's all I can say. I have ended thinking this is a trial and error process, and then you eventually get it.