VISA NOT Required if you fail a flight review
A visa is ONLY required on a FULL TIME training course doing over 18hrs of flying a week.
If you intention is to go to the USA to rent an aircraft and fly then you do NOT need a visa.
If you were not up to speed and needed more flying then as long as you dont exceed the 18 hrs then your good.
Trust me I do this all the time for 61.75 flight reviews.