The manual for the Tower Trainer MKii does not specify a high torque on anything. If fact, they show Futaba 3003 servos which are standard as standard gets. The throttle is the last place you need anything other than standards. You want to move your carb.. what harder if it gets stuck????
The rudder and throttle are the least stressed servos on that type of plane. Elevator and aileron take much effort to move. If you convert to twin aileron servos, then standards are WAYY more than enough, and the elevator is yet still enough. In fact, the stab structure will probably fail before the servo runs out of torque.