Castle Creations motor and ESC combo recommendations for the Losi Rock Rey:

Copperhead 10 w/1410-2100kv - 12T pinion on 3s = ~30MPH
Mamba X w/1412-3200kv - 12T pinion on 3s = ~40MPH
Mamba X w/1415-2400kv - 14T pinion on 4s = ~48MPH

Note: All speed estimates are with stock tire diameter, high quality batteries and appropriately sized battery connectors.
Always monitor ESC, motor and battery temperatures on new setups or when changing gearing or batteries.