I can't really say for the Losi, but on the Xray, I would take spares of:
- The Rear Hubs
- Front C-Hubs
- Front Knuckles
- Front Upper & Lower Arms
And, really that should have you covered, though it is a very strong truck, so you shouldn't really need any of those, (I never broke any parts on my XT8, and I ran it for most of a season). Though the rear hubs are a known weakpoint. The front suspension components are always a good thing to have too though.