Don't use the undertray! That alone will help it all get cooler. I've used 3m clear vinyl for years instead of an undertray for this very reason.
A fan will help but that isn't a rediculous temp for an 8 minute run. I agree that 8 minutes on a mod brushed motor is asking for it. That is simply too long and the motor is telling you this. My other though is that you really need to make sure that you have that motor sprung right. Medium springs will not get it done on that motor in that chassis. You should be running med/firms or firms.
Your gearing is good so just make sure you don't have any bad bearings in the rear hubs,etc.
A heat sink can help as can the fan. However the heat needs to get out of the body/chassis as well. So just blowing warm air over everything won't get it done...