Hi Dave,
When you say the kickstart is jammed - is there any movement of the lever before it jams?
If there is no movement, then the kick start shaft itself is probably seized in the bush. To get at it, remove the shock absorber cover from the gearbox sprocket, then undo the 1/2" nut that retains the sprocket assembly (you may have to remove the screwed plug from the opposite side of the gearbox to access the other end of the bolt). With the sprocket removed, undo the 6 x 1/4"BSF nuts that secure the final drive housing. This can then be withdrawn complete with the final drive shaft and bevel gear. Through the aperture you can then work on the kickstart assembly.
If there is some movement before it jams, then the problem is likely to be with the auxiliary shaft. This is in the bottom of the box and is supported by the auxiliary cover (the small round cover with three screws - just below the pivot of the swinging arm). The auxiliary shaft has a cross pin that locates in slots in the cover - any overloading results in the cover breaking at the root of the slot. This can lead to the gears jamming. I have never understood why Douglas went to the bother of stopping the shaft from turning - the 2 gears it carries are only in motion when the kickstart is operated, so negligible wear would occur if it was allowed to rotate, and the boss could be left solid and therefore much stronger.
Regards,
Eddie.