Keeping that door lock original, is great, but.........
If you replace it with something that's been used for the last 44 years and still going strong, you will not only solve the issue, but you can quit looking for something that no longer exists.
Installing a small plate on the door will enable you to use a Bargmann L-100, or the Bargmann L-100 Special, in some cases.
It's certainly far cheaper that just an after market handle, which means you will still contend with a lock that's 50 years old, even it you replace just the handle.
But, as always, it all depends on a person choice.