Help - Lost fifth gear!

The car is a 1.8 carb 1991. I was driving it down the M4 when it started making a hell of a noise in fifth, and now fifth does not exist anymore. I can physically select fifth with the gearstick, I can hear the shaft within the box spin, but nothing happens (ie does not drive). From looking at the gearbox chart, I think that I have worked out a fews of the basics, but would like confirmation.

The assumption is that the gearbox is dead!

As long as I get another gearbox with a 22mm shaft, I am fine, as I can either get the different driveshafts for it, (A),  the reversing light is easy to resolve and whether it be hole or stud does not matter.

A) If I end up changing the shafts, from 90 to 100, what is the difference at the wheel end, or is this such a pig that it shouldnt be bothered?

All advice greatfully recieved, it is fortunate that she died just before being made cosy for the winter, but need to have it resolved ready to tax for the summer (When the day finally gets here!)

Thanks