As mentioned in the quoted topic, 3D data for the plastic parts of the Mk1 has been provided, so in the unlikely event that something happens, it looks like we can use the printing service DMM make in Japan.
You can choose from a variety of plastic materials, and other materials including gold, silver, aluminum, and titanium(If ignore the cost), but I think PA12GB, a nylon + glass bead material, would be best (similar materials are also used for the stocks and grips of airsoft guns).
As for drum brakes, there’s nothing I can do if the drum body wears out beyond the specified limit, but it seems like you can do something about the shoes by having a brake shop re-lining them. 
As for the BB, I think it should be possible to do something about it if I use a generic Shimano ratchet, and equivalent standard parts for the bearings and races.
The crank is one with the shaft… I don’t think there’s anything that can be done about that… 