I don't know too much about vacuum cleaners, but most trainers can do a bit more than just fly sloppy circles around the field.  Some will fly outside loops in close to stock configuration, but many have difficulty in doing simple things like spins.

The first thing to change is the center of gravity.  Trainers are designed to more or less fly in a self recovery mode.  To do this, the airplanes have the CG moved forward with the decalage to pick up the nose out of a dive without the pilot doing anything.  So move the CG back to the rearmost position shown on the plans, fly and retrim.

Increase control throws seems obvious.

The next easy thing to do to improve the airplane is reduce the decalage.  That means putting the wing and tail on the same angle of incidence and decreasing the down thrust on the engine.  You can also change the wing to another if you really want to experiment (it's surprising how often a wing survives a crash).