Well, the first step would be to use a steel pinion. Aluminium (or Duraluminum) is rather soft and not hard wearing at all. The anodizing (which is hard) is merely a thin layer, so is no match for sand.
At the same time, do keep the plastic (probably Nylon) spur gear. Contrary to aluminium, Nylon is actually really hard-wearing. Also it is soft enough to sort of 'absorb' particles so it actually doesn't grind the steel pinion as much as it would when the spur gear was of a harder material.
Lastly, there might indeed be underbodies. If there are none, reducing the amount of dirt that hits the gears could help enough for a long-lasting spur and pinion
For example: you could make a protective flap that prevents dirt from entering in direction of the wheels or the body. An old wing or a PET drink bottle is the perfect material. Just keep in mind that it is open enough so that dirt doesn't pile up there
