Motorsport is typically written as a single word, especially in British English, where it is the standard form to describe competitive motor vehicle racing (such as car or motorcycle racing). In American English, it is sometimes seen as two words (motor sport), but the single-word form (motorsport) is increasingly common and widely accepted internationally.
In summary, 'motorsport' as one word is the most widely accepted and commonly used form today, especially in the context of organized racing activities.