If you make the changes talked about above, it will probably be able to do it. What is the trainer? Most trainers are set up very nose-heavy by design, which helps learning to fly, but really hurts acrobatics. You'll also need to increase the down-elevator travel.

While you're at it, inspect the elevator pushrod. Most trainers have the pushrod below the stab, so it pushes for "up" and pulls for down. That means you won't have any problem with pulling hard down. But if you're elevator pushrod is on top, and the rod can flex, then you might get solid "up", but not get any effective down elevator when the pushrod flexes. I've seen this a couple of times with planes that would loop inside easily but couldn't push outside.