I cant make out the carby from here, but it sounds like you are describing the victa G3 carburettor. The mower is a Victa Imperial Reel type mower, and usually, when fitted to this mower, the G3 carby was modified at the factory, and did not have the governor vane.
The orange piece on the engine is the cooling cowl, and those did come in that color, and i have seen that colour on that mower.
When fitted to a normal mower, the starter is on top, and the engine points down, therefore the carby is fitted differently (turned 90 degrees) and this puts the plastic vane up, and inside the cooling cowl, where wind is deflected off the flywheel to the governor vane.
on these mowers, the carby had to be turned to keep the float horizontal, and the governor vane is useless as it does not get any airflow from the flywheel.
In the manuals section you will find information on both this carby, and the one that would be expected on this machine.
the engine is a series 70, MKI, MKII or MKIII, factory modified for the imperial, it is from the 70's, i would say somewhere between 1972 & 1974.
I'll have a dig around, i might even have the original sales brochure for this one.
The electrical connection is the cut-out system, and is what stops spark to the spark plug, when you want to stop the engine.
The valve on the cylinder head is a de-compression valve, it is opened while the mower is off, and closes once the engine starts, this makes starting easier, by reducing resistance on the starter cord, and achieving greater starting speeds.
the extra wire connects to the above mentioned cut-out spade, under the vane.
the fuel/oil ratio is 25 parts fuel to 1 part oil, easy mix is 200ml 2-stroke oil in 5l fuel.
the correct spark plug is quite short, i tend to use champion cj8.
