Before replacing parts it's best and more cost effective to prove parts are bad before replacing them. So many things can cause the engine not to start or stop running it is hard to guess at a cause without being able to do some diagnostic tests. An engine needs fuel, spark and mechanical integrity to run. Fuel pressure and volume is easy to check (even "opening" a fuel line while someone cranks the engine over will verify the presence of fuel but won't verify pressure or volume). Ignition spark is easy to verify and after that plugging in a scan tool to the 16 pin diagnostic socket under the dash can verify the operation of a lot of key engine sensors.