Do you find it ironic that reversing polarity destroyed a reverse-polarity-protection diode?
Presumably it really only protects against the
input voltage being reversed. That diode is a DO-213 / MELF package, which comes in a few different sizes. The thing to do is measure the diameter and length, to match against the datasheet of a replacement candidate.
On second thought, the picture shows a diode directly across the battery leads: this doesn't protect against reverse polarity at all, but can be used to limit charge voltage if it is a Zener diode. It would also explain why it blew, as the polarity is actually never intended to be reversed. If it is, the Zener will conduct forward all the current the battery can supply until it overheats. So now you have a different problem: find the voltage at which the battery pack is fully charged, and select a Zener with that same voltage as its reverse breakdown value (V
br).