It is said that undead bats originated from the unholy experiments of a powerful lich in ages past, an effort to replicate the powers of a vampire using bats as test subjects. This manifests itself during the bat’s flight, anyone paying close attention will notice that they become incorporeal for short periods of time. When coupled with their fast movement rate, this makes undead bats quite difficult to hit. Although they have a weak bite, in large numbers they can be deadly as they swarm an opponent and seek out unarmored areas to attack. All attacks are as 2HD creatures.
Undead bats can be turned by a lawful cleric, but again as a 2HD creature. A successful attempt will turn 2d6 bats, plus two additional bats for every cleric level.
Below is the monster stat block, in S&W Whitebox format. Note you can easily adapt this for Labyrinth Lord or for the core or complete versions of Swords & Wizardry by assuming d8 for HD and keeping to 1/2-HD bite damage (so 1d4 instead of 1d3).
AC 3 
ATK Bite 1d3
SP Incorporeal, attack/turned as a 2HD creature