bad spark plug wire.
Internal engine damage.
no gas entering piston chambers or no spark in the spark plugs. I assume the starter is spinning the engine?
As you did not say which vehicle this can only be a general answer Unlikely that a faulty coil would cause an engine to knock. Very possible it would cause bucking, if the coil is failing there would be a lack of spark. This would cause bucking/missfire, which may cause the engine to shake on it's mountings possibly causing a knock from contact with the vehicle frame.
A 1993 Honda xr100r would have a four stroke engine.
The most common problems would be a weak or no spark OR old bad gas.
The intake stroke, number one in a 4 cycle engine.
Worn spark plugs would cause mis-firing or non firing cylinders, both are no good for the engine, replacing the plugs may stop the knock (Could also be a different problem) But you should replace spark plugs as soon as possible.
Fire refers to spark in an internal combustion engine. When a vehicle's ignition switch is on, direct current voltage from the battery passes through wires to the coil which amplifies the voltage and the distributor passes this high energy to the spark plugs at the engine cylinders at a precise time as the piston approaches the top of its compression stroke. The spark plug has gap between its electrode and its grounded body. When the electricity jumps this gap, it makes a spark. A defect in any of these electrical components can cause the spark plug to fail to produce the required spark.
No it isn't fatal, but it would smoke quite a bit, because a two stroke engine needs two stroke oil mixed in the petrol, and a four stroke just needs petrol.
No, not unless your connect the plug wires wrong or one of the new spark plugs was bad.
If the knock sensor failed the CEL (Check Engine Light) would be illuminated and the engine may "ping" or "spark knock" under load. Serious "spark knock" can damage the engine if allowed to continue.