Usually it's enough to just get one of those clip-on heat sinks for brushed 540-size motors. If You can afford it, with a fan. There's enough space under the bonnet. Or, if it's too high, just stick it through the bonnet. Might even look cool.
I use a cheapie from Carson/Dickie/Tamiya. Works fine ,and fits like a glove. The fact that the motor is at the front, open to the elements also helps tremendously.
That's exactly what I used.


This thing was like $12 on eBay and it comes with a fan. I really just used it because it looks cool.
And with the esc fan blowing like crazy right next to the motor, you can feel a strong draft from under the hood.
But this thing stays ultra cool regardless. 40°C is the hottest I have seen it, If the on boars esc/motor temp readout is correct.