SF. What a shame the gearbox is giving trouble at this late stage. As for fixing it, the box can be stripped almost entirely without removal from the frame. The final drive sleeve gear, bearing, seal and sprocket together with the camplate pivot are the only parts that can't be accessed. Takes a bit of gynecological skill, but is a whole lot easier than pulling the lump from the frame. Most difficult part will be removing the selector rod grubscrew. Remove this first, and replace last. This will avoid unintentional damage to the casing if you happen to push hard on the selector rod. Moving the shaft with the grubscrew in place cracks a chunk off the main casting.
If the box is stiff to turn when in neutral or the intermediate gears a possible cause is a partial seizure between the mainshaft and the output drive sleeve. In top gear the box will turn turn as normal as these two parts are locked together when top is selected.
When refilling the gearbox, leave plenty of time for the inner case to take the oil. The inner cover has only a small hole to connect the two chambers, you think the whole box is full to the level plug. Sure it is, in the outer chamber..... but the main case may be almost empty. That's why they wear out.
Swarfy.
