The schumacher diff is the better one, people use it in their Mardaves.
Spares I've used.... spur gears, diff axle if you go for the diff, o rings as mentioned and not a lot else. I've yet to see anyone bend a front axle.
Tyres chunking are the worst problem.
Round off the edges, use a bit of glue when they start to come off (check them every round for this) Make sure your shell doesn't have much chance of catching the tyres and try not to crash...
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