how to recover secret key passphrase?

[ilf]

Over the years, I have used quite a number of keypairs. Unfortunately, I 
have forgotten the passphrase for some of them. But I do know potential 
parts of the passphrase.

What's the current recommended way to recover the passphrase of OpenPGP 
private keys?

The classic John the Ripper includes a tool "gpg2john" to convert 
ASCII-armored exported private keys to a format that john can work with: 
http://blog.atucom.net/2015/08/cracking-gpg-key-passwords-using-john.html

However, to export a private key from the current private-keys-v1.d/ 
gpg-agent key store, I need my passphrase. Which I can't remember.

I would welcome any hints on how to achieve this.

According to Kerckhoffs's principle, this can be public knowledge, in 
contrast to "security through obscurity". But if you feel like this is 
sensible, feel free to answer me directly instead of the list.

Thanks!

-- 
ilf

If you upload your address book to "the cloud", I don't want to be in it.