Brazing would be my preference for a proper repair. For a temporary fix either epoxy as mentioned or I have personally had luck with "würth quick seal" and equivalent product you can buy without an account would be loctite instant gasket. They come in a pressurized tube, and are similar to RTV although not quite the same. A bit expensive but far more durable in my experience. There are likely other equivalent products from other suppliers, those are just the ones I have used.
It's good for all sorts of off label uses. Everything from emergency gooping up of a hydraulic fitting with damaged threads until a replacement can be sourced, to patching the toes of work boots