If it's just for looks,,the existing drum not being 90* to the bore,,but the assembly being strong enough to shoot as it is....
Then I'd increase the dia of the existing drum by splicing a simple thin walled cylinder over it.
Thick enough to allow you to file that extra mat'l to the needed 90* perpendicular shape to the bore so it looks correct.
Leave the orig flats on the outboard end of the drum as is. No need to cover them up. If they need a little tweaking to make them fall in line better with the 90* fix,,that can be done after thedrum is installed.
It won't be much.
The sleeve doesn't have to be very thick,,just enough so you don't file though the wall thickness of the sleeve in trueing it up.
It could be simply LocTite in place or soft soldered on,,sweat soldered on to the existing drum.
Drill the opening in the sleeve for the nipple before soldering/loctiting in place so you know where it is on the orig drum circumference.
Some refitting to the lockplate cut out will be necessary of course as the new drum will be slightly oversize,,but not by much.
All just for looks of course.