Can stem cells repair a torn acl?

No. No. They haven't been shown to repair ACL tears. The tear involves complete disruption of the collagen fibers of the ligament. The ACL goes from a rope-like structure to a disorganized bunch of mop-like endings. They are in complete disarray. Even if the stem cells could reform the acl, you would need some sort of scaffold to direct the new ligament to bridge from the femur to the tibia.
Sometimes. Use of adult mesenchymal stem-stromal cells (mscs) are currently being used for guided injection of mscs, havested by liposuction, mixed with platelet concentrates and placed via guide ultrasound injections. Significant improvement has resulted. Most using fat derived mscs to provide the adult stem-stromal cells to help in repair. Many avoid surgery using the body's natural healing mechanisms.
Yes. Depends on how big the tear is or if the ACL is retracted or not. Check out Regenexx.Com for more info regarding Stem Cell therapy for ACL repair.