For new axles, talk to Moser Engineering (in Illinois) or Mark Williams (in Colorado. They happily do customs.
The stock axles are most certainly NOT cast, they are forged.
4140 is not a bad alloy selection for axle shafts, but is NOT necessarily the best.
Induction hardened shafts are likely the best way to go.
The REAL solution, especially for racing, would be to convert the rear wheel bearings to a 3/4 floating design--peek at an MGA rear for an example of this design. It removes the full alternating stress (due to vehicle weight, cornering loads, etc.) from the outer end of the shaft (EXACTLY where yours broke) and thus solves the problem.
The 3/4 floating design is a lot lighter and simpler than the classic full floating design, and almost entirely as reliable. And, just as with a full floater, the wheel bearing(s) run in gear oil, NOT grease. Another good thing about it.
Any tech inspector/racing organization that objects to such a conversion in any way deserves nothing more or less than a summons and complaint !!
Your break is a classic fatigue failure, and endemic to semi floating axles, particularly in racing applications.
Glad you had an angel riding on your shoulder (TWICE!!!!). It's not at all uncommon for the rear wheel/tire to roll its way out of the wheel well and pitch pole the car over the opposite corner. Many racers have died this way !!
I wouldn't take a GIFT of a used axle for this application, particularly where there's a past history of failures !!