puttygen supports exporting your private key to an OpenSSH compatible format. You can then use OpenSSH tools to recreate the public key.
Open PuttyGen
Click Load
Load your private key
Go to Conversions->Export OpenSSH and export your private key
Copy your private key to ~/.ssh/id_dsa (or id_rsa).
Create the RFC 4716 version of the public key using ssh-keygen
ssh-keygen -e -f ~/.ssh/id_dsa > ~/.ssh/id_dsa_com.pub
Convert the RFC 4716 version of the public key to the OpenSSH format:
ssh-keygen -i -f ~/.ssh/id_dsa_com.pub > ~/.ssh/id_dsa.pub
