The video above shows how the replace a vent on a steam radiator. In this case the air bleeder valve needs to be replaced on a hot water system radiator. One cannot be substituted for the other.

Did you try JB Water Weld? The manufacturer claims it can withstand 4370 PSI and 300F degrees. I have never done this so I can't say if it will work.

To drain the radiator you will need to shut off the boiler and the water feed. Then you have to drain water out of the system. There should be a drainage valve near the bottom of the boiler. You will likely have to connect a garden hose to do this. After the valve is replaced, you will need to fill the boiler with water and then bleed the trapped air out of every radiator in the house. You should be able to find a YouTube video on how to do it. I think the hardest part is removing the old valve without breaking the threads.