How can I make this work? The only cassettes I've been able to find online are either 12-23 or 13-26, the latter is currently on my bike. I would try to block out gears, but that would require two gears to go unused in addition to the screw on the derailer being too short to do so.
Blocking out gears is your only option here, short of some kind of miracle. If you can't do it with the limit screws on the derailer (which seems hard to believe, but okay), then you'll just have to adjust your indexing so that your shifter is set to the highest gear with the chain on the 2nd highest cog.
What kind of crank do you have? Different cranks have different "bolt circle diameters," also known as BCD - if you draw a circle around all of the chainring bolts (that attack the chainring to the crank), the diameter of that circle. Campag cranks use 135 bcd, most other road cranks use 130 bcd, old road cranks and current track cranks use 144, compact cranks use 110. Anyway, your local bike shop can get you a chainring, or any one of numerous on line retailers.
grinding off the teeth would be legal, as that is not a blocked gear. a blocked gear is when you use the derailleur limit screws to block access to gears. they don't want people screwing in their limit screws at the end of a race so that the gears are blocked for the rollout after a race