Thanks for that Mal. If the drum is just pressed steel the welding is a lot easier and the repair technique much better established. I think I'd grind it back around the hole until it had a semblance of the original thickness, patch the inside of the drum by welding in a simple piece of curved steel sheet, then shape a patch for the outside and weld around the periphery. No machining or difficult stuff involved. That explains why it was both so rusted and so thin - cast iron is a much more durable substance than that. I was visualising it having been left submerged in a tropical swamp since about 1960.
That leaves the clutch as the main challenge. I don't really like the idea of a steel replacement. It has to run on a steel shaft, so it would have to have a thin bronze bush where it supports the end of the spring, which is not all that similar to the original design, and being steel it would be easily recognised as a replacement visually, too. Welding on a cast iron blob and machining it to shape might give the best appearance. However I haven't figured out what that big transverse slot in the clutch hub is for: if it is going to have to cope with really vigorous application of a lever, steel would be a much better material than cast iron. The fact that it's already been broken once supports that argument.
