It is a real hassle to change a Plunger Gearbox, but as you have found with the exception of the mainshaft output bearing, drive sleeve, seal and sprocket, plus the camplate pivot, the box can be reduced to the main casing and blind layshaft bush while still in the frame.
That said the blind bush is the awkward one and is prone to wear, along with that end of the layshaft. My suggestion is to swap the casing for the best one, and at the same time attend to the sprocket, seal and ball race as required. These are common across the range. The layshaft and its bush can be done at the same time. All this is in the Forum, just a matter of searching. Some folks fit a sprocket with extra teeth for more relaxed cruising and sod the speedo. Sprocket change can only be done with the box out of the frame.
Select the best gears, pitted teeth will drive OK, but may be a little noisy. Worn dogs will jump out of engagement, so if you have no choice, go for pitted teeth. All the other internals can be upgraded later as required, without removing the box. All threads into alloy are Whitworth, and beware of muck at the bottom of blind holes, Bearings and bushes pop out easily if the castings are given a good warm up, spit dancing temperature works fine.
The big dilemma is how far do you go? Even with worn parts you'll get a rough runner. Swapping the box requires a degree of dexterity and a way of lifting the engine and gearbox from the frame as a unit. The book says it can be done more or less in one piece, Service Sheet 205 shows how. Dead easy if a nut and bolt teardown is on the cards for other remedial work, inspection or renovation. Crankshaft health check, sludge trap and timing bush spring to mind.
If it is a really early flat spring type box, the layshaft pinion and sliding third gear layshaft dogs differ from later design so must be kept as a pair.. The later type is from around 1950. Also you may find the box actually has no oilseal, just a scroll on the sprocket. Later type seal and sprockets are a straight swap. All depends on what the factory were running at the time. Also after all these years nothing can be assumed to be to original spec.
When building the box, remember to line up the oil holes on the gears with those on the shaft splines, and from experience leave the little selector rod grub screw 'til last. Smacking the inner cover into place with a selector rod fixed but reluctant to enter its hole will break away the casting at the grub screw thread.
Swarfy.
