Is this fixable?
Do you have the tab that was broken off?

Seek out a machine shop that specializes in aluminum welding, since it takes a special skill.
They'll need to re-attach the broken tab, or weld on a new one. Resurface the underside so that the entire thing is planed and sits flush, and surface the top side as well (the one where the bolt head sits). They'll also need to make sure the tab is the same thickness as the rest, or the bolt sitting on it will torque and not give a good seal.
It's fixable, but you need someone who knows what they're doing.
Mine was cracked (not broken off). The guy charged me $60 to straighten the tab, weld it on, enlarge another tab that had some of it missing, and then resurface the whole thing. If they start getting up into the $100 range to fix it, you may be better off just seeking out another used one.








