Easiest way in is to drill. Get a good set of bits (at least carbide tipped), start with a bit between 1/16" and 1/8". Drill in the dead center of the keyhole. This will be your pilot hole. Then replace the bit with a 1/4" one. It should go in relatively easily. This should pop the screw on that holds the locking cam in place. Then, use a flat blade screwdriver in the hole that you just drilled, and give it a turn and pull. It should pop right open. When it's open, you can take off the lock and measure the length. It usually is either a 3/4" or 7/8" lock. You can easily order replacements relatively cheap from Bob Roberts.