I think 'throw-away' is right as Richard says.
You can do the swap though, yup, but with mix 'n match parts you'll almost certainly need the lathe for truing, and you may find you can't get one or other or both small steel dowel pins into the bobbin through the drive-end brass cheek owing to the parts not belonging together. On the other hand, sometimes it's a piece of proverbial and no problem at all.
Withdraw the dowels for trial assembly, rather than trying to force the drive-end on with them pre-fitted on the bobbin and forcing the through-screws to align and do up as well. Filing to a 'D section' on half the length of a 'near-fit' dowel can be done, and worst case, there are plenty of mags running around with the dowel(s) missing.
Things to watch out for are internal clearance for the armature (absolutely minimal cuts to get it to turn freely if it binds, because you don't want to increase the air gap), shimming to get not just the right end float, but also the slipring centralised in the HT pick-up apertures after changing a key part, and then it's a real 'must do' to clock what run-out you have on the drive-end taper/shaft. A lot of shafts are a bit bent owing to heavy-handed gear extraction, and others are untrue because of people having had to mix 'n match in the past. More than a few thou run-out at the thin (far) end of the taper (at the base of the thread) is very visible in a lathe at low rpm, but perfection isn't always an option.
There's always a little bit of flex in the mag, usually some wiggle in the spigot that registers the mag to the rear of the timing case when fitting it, and there's always a bit of clearance on the teeth of the pinions in the gear train . . . . . A value judgment, or suck it and see. What would make some people feel unwell doesn't bother others at all!
Good luck with it anyway, Bill