Yes your rat needs the companionship of other rats - I would look for 2 or 3 kittens asap, and introduce them when they are about 9 - 10 weeks old. Use the carrier method
All rats carry myco, which is usually what causes respiratory symptoms, so you don't have to worry about it being caught. Most rats will not suffer any ill effects from carrying it, but some will have life long problems. Even if the cause of your other rats illness was something else, it's now gone, and since your other rat is well, the new babies will be safe.