yum install http://dl.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-2.noarch.rpm && yum install php-mcrypt* && systemctl restart httpd.service
This should be all that is needed if using apache as backend.
You can check Plesk 12.1.18 preview version. Multiple PHP versions are bundled with Plesk. All popular extensions (including mcrypt) will be available out of the box.