I've already polled the Hero robot owners group about what modern batteries a Hero 1 can use
My inclination is to remove the HERO battery, open it up (nearly Dremeling is necessary), and have it rebuilt by Batteries Plus using comparable NiCads with modern amperage. That's what I did for an IBM Simon phone last year and it worked fantastically well. Rebuilt the two halves of the pack with epoxy and then sanded it flat.
This may be a naive answer as it comes only from my own personal experience, but I have HERO 1's here using 4 6v UB5-6F Lead Acid cells. (Mine are older, but they appear to still available. http://www.amazon.com/Universal-UB56F-Sealed-Lead-Battery/dp/B00NR5NYOY) I definitely complement you on the VERY nice job on the Simon pack. But I would recommend against trying to cut into a sealed lead acid cell. Think taking a sawzall to a car battery. :-) -Corey L. Curator, cpmuseum