Been there done that. You might try snipped-for-privacy@gmail.com. He makes adaptor kits to use older 18 volt tools with the 20 volt Li. As long a you still have an old Sears to go with the kit. You could pick a new 20 volt system for batteries. CP
It really depends on the quality of the cells used. My rebuilds are still running fine, I expect to get about the same service life as the originals.
A LI rebuild would have been nice, but then you need the LI compatible charger which I don't have. At that point, it would have made more sense to just get a new drill kit
My first "self" rebuild was a battery pack for a 1990's vintage Makita drill with a 9.6v NiCad pack. I bought a HF replacement pack on sale for next to nothing and dutifully rebuilt the Mikita with the HF cells. It died after maybe three charges. Turned out, the HF batteries were for a wall-wart style trickle charger and my 1-hour Makita charger fried them. A lot of "sub" classes under the NiCad, NMHyd, and LI street names 8^)
The second attempt was with RC car batteries, designed for abuse and fast charging rates. Worked like a champ, but the cost and effort was a wash.
=BR
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