Send me a PM and i'll give you my number, i've pulled apart a few gearboxes in NC29's and things, and have an MC21 gearbox on the workbench if you want to compare.

I'm not an expert, but over the years have had a couple of dramas with gearboxes that taught me a thing or two to look for relating to damage.

There are a couple of relatively simple things to look for, but unless you know what they are you might miss them. Look for bluing of the gears and the ends of the selector forks, rounded engagement dogs, and you also need to remove the selector shaft the forks are on and check it is still straight. Any damage to those and you can have difficulty changing, and the damage does not even need to look significant to cause a shifting issue in something rotating a few thousand times a minute.