The 1" square piece you refer to is called an "ignition module". The module controls 'when' the spark plug fires.
Your carburettor is a G4 Carburettor.
(Always use a good spark plug to test spark)
Is the module loose? Sometimes if they dont earth out on the block correctly they can cause no spark.
Otherwise, grab a new one (they are not that dear) and plug that in, 70 to 80% of the time the module is at fault.
If it is not the module then the only thing that is left is the coil.