I would first make sure the key is warped and not just poor
workmanship.  There are many old pianos that had poor workmanship.
If indeed the key is warped then it would be touching the straight
key next to it if that key is straight.

In some instances the warped key can be plugged and redrilled for the
capstan to sit under the striker or whippen.  _If_ it is warped then
we take the key and place in a jig that has clamps on it to move the
key the right direction.

I use heat from an iron to gently persuade the key to move until it
is correct.  The wood has some give to it and will normally return
a little so we have to try several times to get it to stay.

Don Teach - Shreveport Music Co.
Shreveport, Louisiana