Convert them to lithium,
Motor technology has been the same since 20 years ago and it's still the same in all new drills "DC Brushed Motors"
Up on opening my "Old" drill and having a look at my motor my motor looked very new and the brushes ware far from worn if not still would go strong for many years to come.
On the other end the battery's ware totally on the end of its life.
Step 1: Opening Your Battery Pack
Order a smaller battery then the dimension as it will allow you to move the battery around.
Step 2: Soldering The Power Connector
Always do one lead at a time solder the positive wire then heat stink the connector and then the negative wire and then heat shrink it.
Also helps to add a deans connector to make charging very simple.
Step 3: Closing The Battery Pack
And if your like me then you want a battery pack with no loose wires hanging out then the next step is for you.