I don't do FMQ yet, but have bought a ruler foot set. It's not easy to do immediately. It takes time to learn and to acquire muscle memory for this new skill. Westalee is the major manufacturer of rulers and ruler feet. I think they are available at other places, too, but don't know for sure. Your 15-91 is one of the best machines for FMQ. The vertical bobbin is supposed to make the thread path straighter (I'm repeating what I've read) and therefore better for FMQ.
I'ts unlikely any parts have been changed on your machine. They last forever and rarely need parts as long as they have been oiled and greased somewhat regularly. If you need a manual to show your the oil and grease points, they are available for free download from
ismacs.net.
The ruler foot set is about $40.00, if I remember correctly. You will need the one for short shank machines. The foot is set just above the fabric so that you can move your quilt easily. The rulers are thicker than regular rulers so that they won't slide under the foot.
That's about all I know about the subject other than it's not as easy as it looks. I messed with it a little and realized how much practice it would take to get it to go smoothly as you have to move the fabric at the same speed that your machine is running.
I would suggest you post your other FMQ questions to the main forum where it will get more attention from those who already do FMQ or are learning. Any questions about your machine are best answered here.
bkay