Determine battery voltage
Since you say this device is "old", the batteries were probably NiCd or NiMH.  NiCd will be hard to find, for good reason.  You could take a shot at replacing with NiMH, which have similar forward voltage and charge profile as NiCd anyway.

Fortunately, NiMH don't tend to blow up, like lithium batteries, when abused.  They are more likely to ooze some corrosive goo.