I have an old radiator. Normally when I do not get heat, I would loosen the screw a little so the air could escape. Once I see a little water, I would now tighten. The screw is so stripped I can no longer turn it and it is impossible to get out. I do not know what to do now. Do I need to replace the silver part? I have no idea what it is called. Any suggestions or help will be appreciated. I will include some pics below.

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yeah, just grab that hex part and unscrew it from the bushing.
replace it with an automatic air vent such as a taco #400.. the thread size is 1/8 inch.
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you will need to shut off the water to the boiler and release some pressure to do this.
if you have other convectors like this and you would like to replace as many vents as possible while the system pressure is shut down, they should all be the same.
this type vent does not need to be bled every year. they are automatic. in fact when you put pressure back to the system, you will hear hissing at all the new vents as they do their job. just keep the little cap on the top loose..

You will probably be dealing with hot water, perhaps scalding water. Use caution and get it right, otherwise hire it out.