Long route is to build your own fontset using a service like icomoon and then installing the font to a child theme.
Short route is to use the <img src="my-image-url/image.svg" alt="icon-a" height="25" width="25"> in place of the FA icon markup shown. If you want to use an SVG image, then you will need to install Safe SVG or SVG Support plugins as WP blocks them by default.