The original battery is 6 Volts, 5 Amp-Hour. The replacement is 6 Volts, 4.2 Amp-Hour. You can buy an almost identical battery (but it's now 4.5 Amp-Hour) from Jaycar, or many other suppliers. Here's where I got mine. The original battery weighs 1,258 grams, while the replacement weighs in at 782 grams, quite a saving.
Next step is to cut open the case and deal with the chemical mess inside. Has anybody opened up one of these before? I wonder if a recycling station would accept the contents of a battery in a plastic bag or something.