If you have a copy of your ssh keys, then simply copy the key files to the ~/.ssh/ directory.

E.g.,

cp /path/to/my/key/id_rsa ~/.ssh/id_rsa
cp /path/to/my/key/id_rsa.pub ~/.ssh/id_rsa.pub

Change permissions on file
sudo chmod 600 ~/.ssh/id_rsa
sudo chmod 600 ~/.ssh/id_rsa.pub

Start the ssh-agent in the background
eval $(ssh-agent -s)

Make ssh agent to actually use copied key
ssh-add ~/.ssh/id_rsa

Reference: https://askubuntu.com/questions/786998/how-to-use-existing-ssh-key-on-my-newly-installed-ubuntu