The cowling is remove by pulling out slightly then lifting up. If the knob is lifted at the same time it will be left dangling. It  is only a push fit over the brass tube gas control.

The control adjuster tube  can lift off the gas control when you remove the Heater front panel. When replaced, ensure the control tube is place over the gas control as the sketch.

The two wires from the gas control up to the gas control knob is the 12volt supply for the Red LED lamp enclosed in the Knob. This alights when you move the gas control.

I doubt if the viewing window securing plate could be removed due to the rust! So a soaking with release oil will be neccessary, before the viewing glass can be cleaned.