There are methods of quilting a completed top in sections. The one that pops into my mind is to cut the batting into thirds, lenthwise. You can leave the backing whole, or also do that in sections. basically, you sandwich the middle third, quilt to within about 2 " of the edge, join the next section of batting with a wide zigzag, secure the sandwich and quilt.
I'd be interested in seeing if you can find vinatage machine with a large harp. Maybe an older commercial Consew?