There may not be one simple solution. The Engine thrust angle may be off. If the nose ends up more to the left after the loop, you may need more right thrust. One elevator half may need to be adjusted. Again, if it rolls to the left, raise the right elevator half a little bit. Another easy fix could be to check the lateral balance. ie..one wing may be heavier than the other.

If it were MY plane, I would make sure there is approx 2 degrees right thrust built in, and if so, lateral balance the airplane. If that is fine, adjust one elevator half.

If your flying field allows, fly directly away from you and pull a loop. You could see if your problem is YAW or ROLL induced