I would go for a used gear set if spare individual parts are hard to find.
I suppose the good thing about old Triumph bits is that every part is available from what i have found on mine.
Not worked on old Jap bikes much. The Honda CB750 i bought about 25 years ago is still running and just had a couple of ignition parts since the boy bought it off me. That engine has not been touched after near 50,000 miles.