Don't bin them. Those toothed belt gearboxes can live a new life if you put a brushless motor inside.
My ancient Privateer used to stagger around for about 4 minutes using a 'Mega' motor and 18 Sub 'C' cells. An AXI 2820/12 (half the physical size of the brushed motor) happily swings a 15" prop on 3S or 4S (3200) and has transformed the model. Over 20 ounces lighter and still air times of 20 minutes. 60+ with thermal assistance.
From brushed to brushless