puttygen supports exporting your private key to an OpenSSH compatible format. You can then use OpenSSH tools to recreate the public key.
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