The rear sprocket looks unhealthily sharp, and as far as I can see, a replacement means a whole new brake drum which is riveted (?) to the hub/spindle assembly.
This then means that the whole wheel will need dismantling and then respoking. Or am I missing something?
And then I'll need to replace the gearbox sprocket at the same time, which means taking the whole clutch mechanism off. Sigh, where are those nice simple Morinis?...