If they fit (or if you can make fit) you could put lithium ion batteries in the old packs. The charger doesn't care as long as your voltage is equal to what is being replaced then no issue.
Couple of things with batteries. Like a car battery they usually are charged yp 10-15% higher than the stated rating. So you car battery is charged to 13.2-13.5v.
Phone batteries are charged to 42-4400mv but the battery is 3.7v rating.
Never bothered to see what a full charge output is on a battery pack for a tool, will have to check that.
I set up a damaged flashlight and a Dewalt drill to run off one of those old dc chargers for a laptop or something it put out 19.2v so I figured I could use it on them.