When you use "had" with "if", you are setting up a conditional (possible rather than real) situation, so you need to continue in the conditional form with something such as "could" or "would".
If you had the chance to visit the USA, you would be able to improve your English.
So, since what you are writing here is a future, you will use "have".
If you have the chance to visit the USA, don �t miss LA. (Don �t forget the comma)
Bruce