If you are keeping the stock air box...you can use the stock paper filter, or get a drop in K&N.

I used a very large round foam filter....but I eliminated the stock air box and battery box and used RYCA side panels for my Cafe'.

If you are not modifying the look of the bike....the stock air box and filter does a good job of filtering and keeping water out of the carb in bad weather.  It most likely flows enough air until you start to add cams, a Wiseco piston, and do some head and exhaust work.

The filter I used is a UNI 2-1/4" x 6" long angled 2 stage pod filter.  The part number is UP-6229AST.

Th 2-1/4" is the size of the clamp mount, and I cut the rubber carb boot to the right length and used a piece of 2" ID PVC pipe for a connector between the rubber boot and air filter.  The breather hose from the engine goes into a small box down by the swing arm to catch the oil debris like the stock airbox did - then the breather hose goes back up toe the bottom of the rubber boot just before the carb inlet.   The breather doesn't show in the photo...I added it later.

No....the 2-1/4" filter is mounted to the rubber boot that fits on the carb.....not to the carb.  Can you see the clamp mid-way back on the rubber boot?  That is where the air filter is clamped onto a piece of 2" ID PVC pipe...and the carb boot has been cut in half to mount on the front half of that PVC pipe.