I think no higher than 4000kv or lower than 7.5t brushless.
I have two 2wd sc10s. One on 8.5t brushless, one on 3800kv brushless. On 2s lipo, they only go about 30 mph.
If that's too fast, put a huge spur gear on with a tiny pinion. That'll slow her up, get more oomph on the low end, and keep the temperatures nice and low so he can hammer on the throttle to his heart's content.
Have fun racing!