I have had two English pianos in which the keys did not move.  The
late Frank Holland of the British Piano Museum suggested the following
"fix" to me: simply run an elastic band around the whippen and the key
capstan.  This way when the pneumatic moves the whippen it will pull
up the tail of the key.  On my Duo-Art piano in place of the elastic
I have used a tiny adjustable clip attached to the key capstan or rod
along with a thin thread to couple it to the whippen.

There will undoubtedly be some suggestion that both the loud playing
and the soft playing will be compromised.  In a number of tests I have
run I am not able to hear any difference.  The originality of the piano
has certainly not been compromised and the "fix" is elegantly simple

Phil Dayson