NiMh is pretty safe, ie. won't explode in a fireball. I would charge it at a very slow rate to see if it can be revived. Might want to try something like a C/10 charge for 14-16 hours. If you don't have a hobby charger, then go ahead and throw it on the specified charger. The battery probably won't hold a charge too well, but its worth a shot!
You shouldn't have a problem with it, it's a NiCd battery, which is pretty resistent to overdischarge and should still be able to hold charge like before. The battery isn't NiMH, the MagCharger didn't use NiMH until recently.
The OEM charger is a pretty slow charger, so you should be OK charging it normally.
M*gCharger battery has about 32mm diameter and length of 180mm with 6 Volt and capacity of (2200, 2500, 2600 NiCad) or (3500mAh NiMH) 2x21700 has about 21.5mm diameter and length of 150mm with 7.4 Volt and capacity of 5000mAh
It will be a good idea to switch to 2x21700 if you willing to individually charge them with proper charger. LEDs can take 1.4 Volt extra but for incands there are many long life 6 volt bulbs (5W, 10W, 20W and 30W) to choose from.