I am an international student too. My score is V 161/ Q 168/ AW 4.0. I am applying for political science phd programs and I believe that my gre is actually improving my chances. A 3.0 in AW would hurt my application, but that is because I am a political science major. I think in engineering you can survive with a 3.0, as long as you have a good toefl score to show that you are fluent in English. Of course if you can get your AW score higher, it would be better for you; but if you forgot some stuff about Q&V, and you have to study them in order to take the test again I think you should leave it as it is. Instead get a high toefl score, write a good sop and if you are thinking of applying next year prepare a good writing sample and send it even if it is not mandatory. Show them you can actually write in English and you will be fine. I know that some engineering schools are not even looking at the AW score, also as an international engineering student you have a great verbal score. I think you will be fine as long as the rest of your application looks good.
By the way, what kind of an engineering program would require a 4.5 in A.W. It seems completely idiotic.