I want to extend my thanks to Skippii, the author of this thread. Great "how-to" that has been ~mostly~ successful in my own gas cap rebuild project. A couple of "gotchas" that you might want to watch out for if you have to do this yourself.
Got this bike (2005 Ninjette) off Craig's List as my daughter's first bike. I'm firmly convinced it's been sitting in a shed since at least 2006. Cosmetically, it's beautiful. Though the battery was dead, it took a charge and all the electric stuff works and it did crank.
Couldn't get the gas cap open though. After doing much research I tried everything mentioned anywhere by anybody on how to get a stuck gas cap open. No go. Nothing was going to get the key to turn.
Then I stumbled upon this thread and realized I'd have to go the complete removal route.
Gotcha #1: The OP is right, after drilling a hole to get at the inside screw it is NOT necessary to completely remove the screw. Since he didn't sound sure about that and since - after drilling the hole and unscrewing the inside screw - I could not get the cap to come off. So I figured he was wrong and complete removal of the screw was necessary. What I didn't know was that you can't just lift the cap straight out. You have to rotate the cap a quarter turn counter clockwise. Then it comes right out. So, my mistake was first trying for complete screw removal. My first mistake of many. Though my first hole was nearly as pretty as the OP's, subsequent attempts to widen the hole added ugly on top of ugly.
Bad Ugly Hole
Upon removal it immediately became apparent that absolutely no method whatsoever would ever have gotten that key to turn. The rust and crud had basically welded the locking mechanism.
Condition Upon Removal
The cause was likewise immediately apparent. The entire inside of the gas tank sported some sort of complex crystalline matrix of rust.
Crystalline Entity
Removing those two bottom screws became a nightmare. One I was able to successfully extract. The other? No way. I'm not a gifted or overly experienced mechanic, but I know my way around a wrench. This is far from the first stubborn screw I've ever had to deal with. No extraction method I tried would get the other screw out. I finally had to resort to cutting the head off the screw with a Dremel. After disassembly, it still took a lot of persuasion with a pair of vice grips and lots of penetrant to finally get the screw out. I replaced both with M4-.7x20mm Hex head bolts.
Gotcha #2: Filed under the category of learning from the mistakes of others....
Never, ever, under any circumstances, remove that little flap that covers the keyhole. There are two tiny ball bearings, then a tiny little spring, held by a tiny little copper dowel on each side. These will, of course. come shooting out at high mach speed numbers - never to be seen again. Even if you do, somehow, maintain possession of these miniscule parts - good luck re-assembling the hinge.
Following the instructions in this thread, the rest went fairly smoothly. I now have a working gas cap.
Partial Reassembly - Slathered With Lithium Grease Yes, that's lithium grease - get your mind out of the gutter!
Partial Reassembly - New Bottom Bolts You can see where I had to decapitate the original screw.
Reassembly Complete! Ready for re-installation.
Next up - rust treatment of the gas tank.
Footnote: No cats were harmed in the performance of this maintenance.