Ten years ago, I'd have asked you to sell me the steam knob. They tend to crumble after lots of heat cycles. They're also no longer available.

Jack

Looks like it may be frost damage. If the crack hasn't extended within the O ring area it may be possible to devise a strap screwed into the top of the boiler extending extending to the screw hole in the group to clamp the corner of the boiler with the missing ear.
If you are successful in the repair there are a few other issues to address, e.g. the OPV vents hot boiler water to the water tank, replaced with cool water from the tank. My 2016 OWC renovation thread on the UK coffee forums is still available, which may be helpful.
Good luck.

If you still have this, I'd love to buy it from you, Im rebuilding mine and need alot of those parts.

Let me know,
Thanks