I have no idea how durable the Mi3.5 is, but personally speaking I don't carry any spares when I go touring racing, I haven't broken a part for years - most of the cars now are pretty durable.
Generally, the parts that break are the moulded parts around the extremities of the suspension - arms and hubs. That should cover you for the most likely breakages. But unless you keep a whole spare car, there will always be the day that you break or lose a really unusual part, and that's your race over.
