Tricky autograpraph to authenticate - especially the late ones.
I certainly wouldn't rule this out instantly. Perhaps a touch too boldly-written for this time in his life but wait for other opnions.
Can you add reduce the image in size and post it as an image.
I've had many Keatons and from what I can tell it looks good.
Consistent with his 1960s signatures.
Would need to see it in person to verify though.