Can I export my id_rsa cert from Keychain Access?
There was a similar question on SuperUser here on SE.  

If you have still have access to your old Mac and you can successfully login into whatever services require your id_rsa cert with your passphrase saved in Keychain, you should be able to get the passphrase with no problem.  

Basically, you should be able to see the passwords by selecting Show Password in Keychain.

Have a look at this answer for details