Just replace the bad parts inside. WAYYY cheaper and you can usually get better parts. Oh, and Systema parts are severely overpriced.
Just replace the motor, bushings, wiring, piston head, cylinder, cylinder head, nozzle, and correct AOE.
Piston head: $6
Spring guide: $8
$98 from clandestineairsoft.com
Even with all these parts with a few add-ons, it's still less than half the price of one of those gearboxes.