well I started playing with this same subject a couple nights ago. the throttle body is 2" the intake tube inlet at the air box is 3-1/4"
I put an automotive filter on it and it fit fine, however I didn't like how heavy it was (heavier than the stock airbox), and it was right in the chain line.... So anything you fling from your chain would hit the air filter..... . I've been thinking about keeping it stock for a lil while.
But if you wanted to, you could try a 2" 10degree bend flanged K&N and bolt it right to the throttle body. I kinda like the thought of this since it exposes the upper shock adjustments and tucks the filter up away from the chain line.
Here is a link Rob sent me btw: