you need 3 things to start an engine: 1) the engine has to crank; this uses the starter and associated wiring, switches, and relays (solenoids).
2) Gas or fuel; this entails the carbuerator or fuel inject systems to be in good order.
3) Spark; has to happen at the right time in the right sequence of events and includes the spark plug, spark plug wires, distributor cap, and rotor (if so equipped) and a condenser for points type ignition systems. Also the ignition coil(s) must work. They produce the high voltage for the spark.