As noted above, the 50.4mm 5-pin design was common and interchangeable among many brands. Cottered BB spindles do come in a variety of diameters, though, so it is not a foregone conclusion that any cottered spindle will work with whatever 5-pin arm you find.

Is there a compelling reason to use a cottered crank? By the 1970s these were only found on relatively low-end models. A cotterless 49d or even a TA Pro-5 would work fine on your Motobecane.