Hmmm... I haven't broken one of those yet. Three options (out of many more
(A) Replace just the bad motor and the bad gear. Individual gears are $12 on somewhere like this on ebay
. A motor is another $17 or so when bought standalone.
(B) Get a whole new gearbox + motor for a model with the same # of teeth as the bad motor. Use the new gearbox in place of the one with the broken gear, and use the old motor (from the gearbox with the broken gear) in the gearbox with the busted motor. I think the DUne Racer is 16T (should be printed on the gear on the motor), which is the same as the F150, so a gearbox like this would work
. Biggest advantage with this is you'd have some spare parts, since you'd have an almost-complete gearbox sitting around, in case one of the other gears breaks.
(C) Get two new 23T (Corvette) gearboxes w/o motor, drill out the axle hole, and put in 775 motors with 22T gears. You can go with the super-cheap Nichibo 7013F
($16 for 4, shipped), which have about 30% more torque than stock motors, basically the same speed, and pull only about 10% more stall current. They should last quite a long time at 18V. But I recommend you only run them in a 21/22/23T gearbox or with an ESC, because otherwise they put too much stress on the gears at startup (22T vs. 16T means about 40% less stress on the teeth, but also 40% more speed; since the 775 motors are 30% higher torque, that means overall about 10% less stress than the old stock motors put on the current gearboxes you have). It may end up being faster than you want though. You'd have lots of spare parts, but it's substantially more work.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors