A used backup machine would be helpful, however no two machines sew exactly alike and your seams may be off by just a hair and you will have a noticeable difference after you sew a lot of pieces together. I try never to use more than one machine on the same quilt, but sometimes you have to make do. Do you have a friend who would loan you a machine until you get yours back? If you have a senior center check to see if they have a machine you could use. If that machine will be gone to the repair person for so long he should provide a loaner.