Extracting the public key from the private key
The -y option of the ssh-keygen tool outputs the public key:
ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub
The comment of the public key is lost so you need to edit ~/.ssh/id_rsa.pub and append a comment to the first line with a space between the comment and key data.
A truncated example of a public key:
ssh-rsa AAAA..../VqDjtS5 user@pc