Lost GPG private key passphrase

[Daniele Nicolodi]

Hello,

I have an oldish GPG key for which I have lost the passphrase. I have a 
very good idea of what the passphrase is constructed but there are some 
characters substitution that I must have used back then really escape my 
memory now. I think that a tool like John the Ripper could make easy 
work in retrieving it.

Does anyone know how to run john on a private key stored in the format 
used by the new keystore used by gpg2?

Thank you.

Cheers,
Dan