I put a cone filter (£8 on ebay) on mine, made a big difference, quicker of the mark.
I just cut off the old air intake about 6" from the air box, and it was a nice fit with a jubilee clip. Put a smaller cone filter on the smaller inlet and removed the old filter from inside the box.
Try it, you'll be surprised......makes a decent tone as well....barp, barp.
NB. There's a piccie further back on the old what did you do thread, maybe 10 pages back.