This might be helpful http://scifiwire.com/2009/01/terminators-anton-yelchin-talks-about-being-kyle-reese.php
Anton Yelchin is a good actor (you can check him out as Chekov in the Star Trek movie - which I was luckly enough to watch last week) and we've had many actors replacing others for characters in the Termiantor series, so I think whether he resembles Biehn or not shouldn't effect his performance or the film itself.