I like the mallet idea, but to date have done a few of these things by scoring along the seam lines with a knife or razor saw, then taping the thing back together to facilitate easier rebuilds next time. But if I were actually going to use the thing regularly? Forget the expense of rebuilding a stock battery: Open Portable housing, remove stock gold-plated battery terminals from battery bay (they are clipped into place and wired in via Fastons). Build new battery leads with spade connectors on one end, and Fastons on the other. Snake them through suitable opening in the battery compartment. Now just use any suitably-sized 6V battery with Faston-type connectors. Best of all, these mods are fully reversible. Had previously connected a D nicad pack that way and it worked great. NIMH should work too, but I probably wouldn't leave them sitting on the charger longer than I had to.
2V 5 AH Enersys Cyclon batteries are still available but kind of expensive and neither Mouser nor Digikey sells that particular size.
I've used the 12V version of this to revive a dead AGM car battery:
http://www.batterytender.com/Chargers/Battery-Tender-Junior-12V-0-75A.html
But did have to clean up a puddle of acid afterwards: I guess the process must've been causing the battery to bubble with hydrogen gas. Fine for a wet cell with vents but I dunno about doing that with a sealed battery.