Gas hot water heaters are a great source of parts for burners. My original liquor tun used the entire assembly. I welded a 3/4 inch coupling to the keg at a location so that the flame spreader was four inches below the bottom of the keg. It worked great.
Later I wanted to control the liquor heat with with my PC so I removed the gas water heater control in favor of an AC controlled furnace valve. A thermocouple well now fills the vacant hole.
If you use this approach you'll need to get the control to work at higher temperatures. The upper end is normally limited to keep the hot water supply from scalding. Normally there is some kind of stop under the knob. A pointer on the adjustment shaft needs to be removed and simply replaced in a different position. This allowed me to boil the liquor if I wanted.
You should note that there is a difference in propane and natural gas setups. Try to find the correct type to start with. I believe the primary difference is the orifice size. Natural gas being much larger. The threads that attach the burner to the valve are left hand threaded for propane and the normal right hand threads for natural gas.
After unthreading the burner and pilot lines, you simply pull the assembly from the old hot water tank.
After the burner is removed you need to unscrew this control valve from the old water heater. I need a large pipe wrench to do this. If you don't have one and are feeling lucky, screw a 1/2 inch NPT pipe into the gas supply inlet and use it for leverage.