1) The only company I knew of that would stretch pre-owned machines was called Wow, and they went out of business several years ago.
2) I actually owned an old Singer that had been stretched by Wow several years ago. It worked okay, but I quickly became frustrated because it had no stitch regulation, despite the fact that it had an 18" harp, it was also a short machine which meant I was still limited to the size of top I could quilt.
3) I believe Hinterberg is no longer producing new Voyager machines. I think you can buy refurbished ones, though.