The stock 919 lower is cast steel and surely of a low grade, I doubt very much it will be alloyed at all, and most likely a simple carbon steel.
The 919 stem is likely a non alloyed carbon steel, something like a SAE 1020.
4140 is a chrome moly steel and overkill for the stem BUT there is so much 4140 kicking around that it is likely easy to get it cheap, or even to be able to scrounge a suitable piece for free should you have some shop connections to tap.
1340 is a manganese steel that would be a good choice, again, overkill, and not nearly as easily scrounged as 4140.
Curve ball though.
The stock 919 stem, and surely the 929/954/RC51 is hollow.
If you make a stem from bar, it will weigh lots and anyone doing a RC51 conversion will want a stem no heavier than stock.
Long drilling or long boring a length of bar will not be pleasant, especially in an alloyed steel.
You might be able to find a suitable tubular steel that has enough wall thickness to accommodate the various ODs required along the length.
No wonder people want to salvage a stock 919 stem and re-use in a 929 lower.