Keep your engine tuned. A fouled spark plug or plugged/restricted fuel injector can reduce fuel efficiency as much as 30 percent. Inspect the engine's belts regularly. Look for cracks or missing sections or segments. Worn belts will affect the engine performance. ...
As the build progress, you wind up coming back to the engine to add new fiddly parts - spark plug wires (which are made from provided black vinyl tubing, but have to be painted blue), an intake manifold (which has to connect with an inlet on the rear body panel much later)...