I believe that's cast aluminum which shouldn't be an issue to weld with a TIG or MIG, more of an issue will be getting to it with the welding equipment if your going to try and weld the broken tab back on. In order to get a good dependable strong weld you really need it welded all the way round, including the back side, that means your going to need to get the swingarm out of the way. I wouldn't suggest trying to reattach the broken piece because it's broken midway thru the thread and it will be difficult to get the threads to line up between the 2 halves, you can try clamping the broken piece to the frame over a screw threaded in place to try and get the threads to line up, but that doesn't always work out so well and often times results in the tapped hole being sloppy because the broken pieces didn't fit back together like they need to.
The other option is to build up the area with all weld and re-drill a new hole using the rearset as a guide to the holes location. I'm a Tool & Die Maker so I personally would go this route, but if you don't have access to metal working equipment (or know someone who does) your going to have to improvise and figure out a way to machine the weld flat and parallel with the other existing tab. Of course if you find a welder who is also a machinist or fabricator they should be able to also do that for you.