Each part of a motor is 'essential', as nothing will work if all parts aren't present. What is essential is a strong magnetic field in the air gap, as this determines the torque developed -but this isn't a 'part'.
Surging is often fixed by replacing the fuel filter. A simple job and most small engine shops will have the part.
The Cerebrum is the most essential part of the human brain.
Bile salts
Making money
Calyx is the most essential part because it protect the flower
Spark Plug
The most important parts are the shocks and the motor.
Jellyfish do not have a skeletal bone structure, which is essential in mammals.
Primary Motor Cortex
Oil changes are the most essential part of the car and so is the tune up. If you don't change your oil every 3,000 miles as stated in the manual, I guarantee you that you will be purchasing a new motor.
For the most part yes. Unless it is a 5.0 H.O.
Commutator and brushes.