That is not a pulley system!
The old Proctor (not Seldén) masts used to be waterproof. Those masts had welded chambers to hold the sheaves. The halyards in those masts used to run through the luff channel together with the sail’s bolt rope. The big advantage of those masts was that they floated and thus helped to prevent inversion. Obviously you need one big sheave or two small ones, to span the distance from the front of the mast all the way through the mast to the bolt rope channel at the back of the mast. And that is what you see, two sheaves to guide the halyard from the front to the back.
Those old masts are said to have better bending characteristics as well, and are most definitely worth saving!
Maybe a welder could fix the protruding part on your mast? Aluminium is perfectly weldable though it needs specialised equipment and an experienced welder.
The sheaves are riveted to the side plates. Rivets can be removed by drilling them out. Then find some good replacement sheaves of approximately the same size and two new rivets. Any metalworker should be able to get that job done in a jiffy but it is not out of reach of a DIY person either.
If all else fails have your local smith make two Aluminium sheaves on his lathe. Aluminium sheaves are good enough IMHO. Though stainless steel would last longer, your current (Aluminium) sheaves have lasted for 40 to 50 years. Finally, I don’t see why modern hard wearing plastic sheaves can’t be used. I would probably choose aluminium rivets for they agree galvanically with the mast and (Aluminium) sheaves.
I don’t think it would have to be an expensive job but it may take some time and effort to find the right materials and craftsmanship if you are not in the boating business.