I'm never a fan of remaking something as I've found that it's easier and less frustrating to start fresh than to piecemeal something together. The quilts that I make are barely 3 dimensional; store bought comforters are usually pretty fluffy and I'm wondering how you would get a fluffy comforter under your sewing machine needle? Even a walking foot has its limits. The way I would do this is remove the sports fabric, turn raw edges under twice and sew using either a 1/8" or 1/4" top stitch on all four sides. I wouldn't bother with new borders. I'd let the new comforter fabric act as faux borders by centering the sport fabric onto the new comforter and pin, pin, pinning it in place. Try to machine stitch. If it's too much for your machine and your stitches aren't looking pretty then hand stitch it using a slip stitch or any stitch that you can barely see. I would hand tie the quilt to keep all the layers together. If you think of it, post some before and after pictures. I'd love to see what you come up with and how you achieved it. Best wishes!