A lot of the Stuart engines used to come with malleable iron crankshafts. in some cases, like with the double ten, the two throw crankshaft was made by welding two of the single throw cranks end to end, possibly by friction welding since it was quite a tidy weld. That didn't stop the one I had being harder than carbide tooling, so in the finish I made one from solid. I think that in later years they did not have access to the malleable iron process, I have seen at least one where the blank was supplied partly machined from solid.
My triple did not come with a crankshaft, not too surprising since all it came with was the baseplate. So I made the crankshaft from solid. That was a bit of an epic but it worked out fine. Most of the material was milled away with the job between centres on the milling table. That is quite a useful technique for crankshafts.
Once the shaft is made, you can mill away part of each disc to leave the counterweights.
John