Looks like it might be cast aluminum, which can be a pain to weld.
Personally I like welding cast aluminum and seem to be good at it, but a lot of people have a hard time with it.
Do you have access to a TIG welder? You would need to v groove that cracked part fully to get full weld penetration. Also clean, clean, clean with aluminum. Should be bright and shiny and a wipe with acetone wouldn’t hurt.
Alternately, if you have a MiG welder you could take some measurements, trace it and make templates and recreate the piece from steel.
Will be a little heavier but would be stronger, which for brakes might be a good thing.