Create a new user with sudo rights on Ubuntu

adduser ian
usermod -aG sudo ian

adduser asks for a password and creates the home directory. The -aG appends the group; without -a you would replace all of the user's groups with just sudo.

Copy root's authorized key over so you can log in as the new user with the same SSH key:

rsync --archive --chown=ian:ian ~/.ssh /home/ian

Test ssh ian@server and sudo -v in a second terminal before you close the root session.