I just wanted to share a nice looking low profile K&N style filter source. It fits on a stock briggs pulse jet carb and should fit a Mikuni but haven't checked it yet. The website is ADA Racing, the over height is 2", here is a picture of it on a briggs and a link to the site.
Pocket Bike Parts - Chrome UFO Air Filter sp203801 - ADA Racing
Pocket Bike Parts - Chrome UFO Air Filter sp203801 - ADA Racing

