converting Nimh craftsman 19.2V drill to li-ion?

So I have a ton of 19.2V craftsman products, drills, drivers, saws, etc. The battery performance sucks, and have at least one dead battery. Then a couple years ago I got a li-ion 19.2v battery and wow - lighter and way more power. I'm not sure what cells they use, but I was debating cracking it open to see how its made, and maybe trying to replicate it using my dead nimh pack's housing. Has anyone else done something like this? 18650's? 5x series? 2 or 3x parallel? Any circuitry or balancing resistors or?

im interested as well

I too have a few low (almost dead) 19.2 (non li-ion) batts that i like to convert

but i did over the blk friday deals score a li ion battery from sears