Swapping a step down converter
Nicely done photo - I wish tehy were all that good. 

That's not "missing a leg" - it has been blown apart - presumably by electrical failure. 

There is a reasonable prospect that there is other damage. Datasheet here.
Look for damage on lines connecting to pins 2 (input) and 3 (output) BUT could be anywhere.  

The capacitor equivalent to C2 in datasheet MAY be short.