On an all brass model you would need a soldering iron but better to have a resistance soldering tool if the part is on the boiler. All depends how large it is that will determine how much heat you need. Then you might be left with a hole you have to fill somehow. Similarly a brass part on a diecast boiler is likely epoxied on and will leave a hole. Some areas you might get away with just a touch up brush, others might need a complete respray.
Pete