A couple thoughts on this question. First, I doubt any streamliners are going to use conventional forks. Upright bikes with lots of streamlining probably would. In that case stock forks would be fine as long as the springs were set up for the weight of the bike. I think a bigger concern for the really fast bikes is the tires and making sure they can handle both the load and the speed. At high speeds there just isn't a lot of margin for a bad tire, that's why they are inspected so closely. I just looked at one of my Bonneville bikes and even though the tire has less than 100 miles on it, there is a slit about 4" long down the center, guess that is my new dyno tire.
