If the selector shaft and camplate turns OK and isn't too loose then you can leave it assembled.

Check for endplay - there should be thrust washers on either side of the centre web as shown in the photos.

There may also be a washer under the circlip that hold the selector cam on. This isn't shown in the gearbox diagram, but might be necessary to control the end float.

The thin thrust washer (041389) goes on the inside. The thicker thrust washer (041390) goes on the outside (under the selector pawl). There's also a thin washer (041323) under the securing nut.

The thrust washers are sometimes missing. They are just washers, so you can improvise replacements. The sizes are: 041389: ID 17.9mm OD 25.4mm T 0.8mm 041390: ID 16.3mm OD 24.4mm T 1.65mm