Andy,
The servo band brakes can be the bane of the Douglas owner. Some examples through minimal maintenance work great, others with tinkering remain poor performers and dragging your feet on the road would have more effect. They can work very well indeed, but get a few small things wrong and it can seem hopeless.
I would suggest starting out by checking these prior posts on the forum, as they might focus your general question to more specific avenues to investigate.
Article on the servo band brake.
https://www.douglasmotorcycles.net/index.php?topic=338.msg969#msg969Relining 350EW brakes.
https://www.douglasmotorcycles.net/index.php?topic=2287.msg8267#msg8267General comments.
https://www.douglasmotorcycles.net/index.php?topic=889.msg2907#msg2907https://www.douglasmotorcycles.net/index.php?topic=1927.msg7072#msg7072Trails and tribulations of another 350EW owner.
https://www.douglasmotorcycles.net/index.php?topic=2197.msg7963#msg7963Brake arm and shaft.
https://www.douglasmotorcycles.net/index.php?topic=2440.msg8823#msg8823https://www.douglasmotorcycles.net/index.php?topic=3431.msg12530#msg12530As for the kick start conical spline shaft, you have to be careful as Douglas made many variations of these. Different length and diameter of shaft; and the gear teeth changes from standard to stub, and then to coarser pitches. As others have pointed out here on the Forum, a common mistake by owners is to re-fit the kick start lever to a more upright position so as to have a more generous swing. This is a mistake as the travel now allows the quadrant gear to slam into the internal stop (which is one of the cover studs. This bends the stud, breaks the casting around the stud hole, or loosens/strips the conical spline. Or all of the above. The proper return position for the lever is just slightly above horizontal, as shown in period EW catalogs and literature.
I have not heard of these quadrants being re-manufactured. You could look for a better example, as 350EWs are not that rare (noting comments above.) The alternative is to have a new shaft made and applied to your quadrant gear (if the gear is still in good condition.) Other than the milling of the conical spline teeth, this is a straight forward lathe project.
-Doug