Remove the filler neck by removing the three or four screws visible under the gas cap. Pull up and out.
Remove the screws holding the bottom portion of the flapper to the filler neck. As I recall, there are three. Retire to a high cliff and toss over your flapper. See how far it goes. That should leave a rather large hole in the filler/cap assembly for you to pour in your gas.
Find the vent line that runs from under the tank to the front top of your crank case. Cut it about an inch above the crank case and plug with an old golf tee or a machine screw. Route the other end (still attached to your tank) overboard so that any gas draining through it doesn't drip on anything important. Leave the other line alone.
If you want to change the place where your gas level light comes on, now's the time. Reach into your tank through the hole and bend the wire arm that runs from the sender unit to the float. It works just like the one in your toilet. Bending the float up lets the light come on later. Since the above modification will let you use at least 4.8 gallons of fuel before running dry (I haven't pressed my luck beyond 4.8 gal.), you may want to set your float so the light comes on at about 4.2 gallons of use. You'll probably have to experiment to find the best setting. BTW, I found the rod very difficult to bend, even though I was able to get BOTH my child sized hands into the tank. So don't worry about over-bending it first time around.
As an aside, I *always* can go *at least* 200 miles on a tank of gas, and have gone as far as 260 miles without running out. That damn idiot light is just about useless as a gas gauge, much better to rely on the odometer. In addition to coming on too early, the light also seems to come on sooner in the winter (maybe because mileage drops with winter fuel blends, or due to lower temperature), or it comes on, goes out, then comes on again. Especially annoying if it stays out after the bike has been parked, only to come on again 20 miles later. Well, am I really out of gas, or is that stupid light just f**king with my head again? YMMV
Scot J. Marburger firstname.lastname@example.org
"There's No Space Like CyberSpace"