I agree, brake drums are usually made from cast iron as it has good wear & heat characteristics but it is unweldable in this situation.
Cast Iron can be welded but it is fiddly to do, it will be harder to do with aluminium on the opposite side of the weld and the nickel rods required to weld it with will make a hard spot that won’t wear the same as the rest so is unsuitable for a brake drum.
The broken part seems from the photo’s to be superficial to your needs so I would also say break the piece out, you don’t want it coming out by itself while your riding it, the results wouldn’t be nice.
You may even damage the pipe.
Be sure to chamfer all edges around the break