How to add sphinx engine to mysql 5.7?
Install MariaDB which will run on top of a MySQL-5.7 data directory. This has SphinxSE included in every mariadb-server package on Ubuntu.
You will need to install the plugin:
INSTALL SONAME 'ha_sphinx';

Please check the incompatibility documentation and make a backup before upgrading.
Note there is little effort being put into maintaining SphinxSE (note MariaDB is a few versions behind the latest 2.11, only minor differences looking at code), and that it has been dropped from SphinxSE 3.0 onward.
I agree its a beautiful piece of software, and I know of one large production use of SphinxSE from a former client of mine who's websites are still running and appear to be using it still.