Motors coming loose in their mount is not uncommon. I used to have this problem when I started racing some time ago. These days I don't take any chances and use spring washers under the motor holding screws and can say this is one incident that never happened to me in a race. Most peopel don't use these, I am not sure why but every now and then they have the motor come loose in a race and it's annoying.

Thomas, Tamiya's CR01 uses a lot of threaded rods which are 3mm. They're not that long though. There is one part that is long enough (perhaps longer than you need), but it's soft aluminum. It is the steering rod on M05 mini cars from tamiya. The stock kit part might actually be a better idea as it might be steel.