Is this Radiator handle/valve a replaceable part? Do I need to drain the system first?
I have lived with hot-water heating for 70+ years.  I have never seen this valve used.  I have been strongly advised to never touch them, since the packing washer will have hardened decades ago, and turning the handle (if possible) could produce a permanent leak out of the system around the stem. All my systems have been one boiler-one circulating pump, feeding all the radiators, and one thermostat to switch the boiler on/off...

If you chose to remove the valve, you will need to shut off any make-up water supply, and drain all the water from that part of the system at or above the level of the radiator valve. 

It is quite likely that you will need to loosen the large square fitting to access the washer that shuts off the flow of water through the radiator.