Here are the ways to get most of the secret accessories:
To get the Top End S engine, you must finish within one of the top three places on any track in Class A, and at least one of your lap times must end with the numbers 99.
To get the Top End SS engine, you must finish first in one of the top three places on the harbor track, with at least one lap time of less than 42 seconds in Class A.
To get the Acceleration S engine, you must place first on the Caves track in Class A.
To get the Acceleration SS engine, you must place within one of the top three places on any track in Class AA while equipped with the extra Lights 2 and the weapon Missile 2.
To get the Spoiler 4 part, you must finish within one of the top three places on the Alpine track in Class A without using any of the Dash zones.
To get the Bumper S part, you must place within one of the top three places on the Ruins track in Class A without driving on the grass.
To get the Light extra, you must place first on the Lakeside track in class A.
To get the Air Duct extra, you must finish within one of the top three places thirty times on the same course in Class A without turning the game off.
To get the Nozzle extra, you must finish within one of the top three on any course in Class A using a non-hidden extra.