When you join the LDMCC, and order you kickstart return spring, note you need to ask for the 'three-speed' return spring, as there is a 'four-speed' kick start return spring available too, which is very similar. But if you say it is for a EW, you'll get the proper spring for the three-speed gearbox.
If the bearing at the center of the of the clutch is worn (that permits the disk to rotate on the carrier sleeve, which in turn slides on the flywheel hub), the pull of the chain will cause the clutch disk to tilt, allowing the periphery to drag on the flywheel back plate. Then the clutch will not completely release and you get horrid sound effects like a rock crusher when trying to engage a gear.
I have not heard of kickstart quadrent gears being available. I made one for my F28 (a 1928 600 EW) by cutting a full gear blank, then sawing out the quadrent from that. I may have a spare blank about if I've not given it away already. Be aware that at some point Douglas did change the size of the teeth (made them larger) on the kickstart quadrent, but I think this was on towards 1931 and not during the EW era. But they used the same three-speed box right into the mid-thirties on the cheap models, so beware if you are offered a latter box. You will need to swap over both the quadrent and the pinion together.
You will find it a tad more sedate than the CB600! But very lithe to ride.