60 years ago this would have been a simple repair, just pull off the broken bit, make sure it fits with a little filing, clamp it back in place and use a little silver, like easyflo. We used to add Campagnolo gear hangers, and this isn't much more. A penetration joint, not weld, would be more than adequate. My half tin of easyflo flux is still viable after 60 years.
If it was my frame that is what I would do.