Firstly, you obviously have problems with the clutch, so the engine and gearbox will have to be separated. There may be some mechanical damage to the plate, or the clutch may be lifting but not returning - this was experienced with the introduction of the early generation asbestos-free linings. The linings gave off a fine dust that tended to accumulate on the drive pins through the flywheel, thus jamming the pins and preventing the clutch returning - the more modern woven linings don't suffer from this problem.
Steering head bearings - the bottom bearing is a commercially available taper roller bearing - the top bearing is a Douglas special that screws on to the steering column (I don't know of any alternative that will fit).
Exhaust pipes - the pipes for the normal barrel silencers have a flat 180 degree bend - the pipes for the earlier 'woffle box' have the same bend but with a twist in it, so that the pipes enter the box at an angle. These pipes also have the ends blanked off and slots down one side for the exhaust to escape into the box. They also have a boss that bolts up to a frame lug to retain them.
Regards,
Eddie.