Having just rebuilt a couple of old train engines, they all are missing some parts. It would be fantastic if PM could release train packs. Like 3 of them. They would contain commonly missing parts for the old engines.
My suggestion would be the 4051 engine part pack. It would contain the roof, smokestack, tools, controls from inside the cab, the light housing, the horn, the connecting rods and pins, the buffers, and the side stuff that mounts to the boiler. Charge $20.
Then make a smaller set for the diesel engines - just roof, horn, light housing and tools. Charge $15.
And lastly a more expensive set that encompasses the 4052 and 4054 engines and coal cars. Charge $30.
Just the plastics, not the engine rebuild stuff, so it would be parts playmobil has the molds and control over.