That V-brake mount is knackered and it’d be too expensive to reweld it. As a last resort you can see if you can use a bolt extractor and a lot of heat to get that boss fragment out but it’s gonna be tough and there’s a good chance your boss was press fit or welded and not screwed in. If you are able to get that fragment out then you can replace it with a cantilever stud:
The cheapest and most realistic option is to mount a caliper brake on your seat stay bridge. You keep your old wheel. You may need to find long reach calipers, modern ones may have too short of a reach or aren’t wide enough for your wheels.
Another option though much more expensive is to get a rear disk brake conversion kit. They attach to your rear triangle. They’re more expensive as you need the conversion kit, the disk brakes, maybe new brake levers, and most likely a new wheel that is disc compatible. It’s your rear wheel so you lose traction before anything else so you won’t brake faster but you’ll at least look cooler.
