Testis have two functions. It produces the male hormone needed for secondary sexual characters and libido. Secondly, this is the place where sperms are produced. While testis would be capable of producing male hormones irrespective of their location, sperm formation is a very sensitive function and may get damaged if the environment is not right. It seems that your doctor has confirmed that your testis are irreparably damaged, and would not be able to produce sperms. While an artificial testis may be implanted for cosmesis, its transplant is not done. Only testicular transplantation in medical literature is retrieved frozen germ cells transplanted to the same person later on, as may be needed in patients requiring chemotherapy. Transplantation of testis from a different donor has many technical and ethical issues. If you want some one else’s testis to produce sperms in your body, why not take his sperms for fertilization instead as a donor, a simple straight forward procedure. For obvious ethical issues involved, the sperms for fertilization are taken from a large donor pool from the sperm bank.