A similar situation exists with Douglas motorcycles of the twenties and thirties, though on rare occasion they will call out a specification relevant to the time. So 5% nickel steel might be given as Hughes & Johnson X84 (proprietary), or S82 (a post WW1 aircraft spec.) Cross comparison, when you can find charts, would show them equivalent to EN39b. They started using material similar to EN36 going into the mid-1930ss.
But the only way to really know is to have a sample item sent to a metallurgy lab and tested. Usually this is an optical emissions spectrometer, where they vaporize a small patch of the surface material. Then you get a precise record of the chemical make-up and percentages there of. The lab may tell you if it is similar to a current material, otherwise you have to find the nearest match using a metals handbook.
Chemical tests on ferrous materials were running $30 in 1997 here in the USA. I think the price doubled since the lab I used changed ownership! Fortunately (?) there are an abundance of broken Dougie crankshafts and connecting rods about to test.
When I made new connecting rods for the Dirt Track Dougies, the nearest equivalent I could find was SAE9310. The modern steels do not have as high nickel content (4.25% is usually the highest offered), but they more than make up for it in purity, constancy, and refinement. Most of these specialty steels are made for the aerospace industry, and they do not tolerate variation, or slag inclusions! The bar stock is invariably vacuum re-melt, Timken of bearing fame does quite a sideline in specialty steel and have a good website with lots of steel spec info. Also available now is more precise control over the heat treatment process, with furnace temperature control to within two degrees and inert atmospheres. Something not available back then at all is cryogenic tempering of ferrous parts to -300 degrees F. In essence this extends the tempering range. The upper limit is fixed by drawing the hardness out of the part, but the lower limit use to be room temperature, but now it is much lower. So you can cycle the part through some seven hundred degrees rather than just say four hundred of old. End result, more refinement of the metal structure during tempering. And it is not that expensive; around $1-2 per pound processed, or fixed prices for popular items like connecting rods, crankshafts, brake rotors, and target rifle barrels.
For highly stressed case-hardened parts SAE9310 is the best with SAE8620 almost as good but far less expensive and more readily available. For through hardened parts with the maximum shock resistance, SAE 4130, or for thicker sections SAE4340, or the ultimate and hardest to source 300M (a modified version of SAE4340) are usually the best choices. It is a pity you can not case harden some of these through hardening steels, as they have some awesome tensile and impact properties. As always, you have to match the ideal steel with the specific conditions it will be used in, and there the lab or the heat treatment firm can often provide some free advice.