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

Copperhead 10 w/1412-2100kv - 12T pinion on 4s = ~45MPH
Mamba X w/1412-3200kv - 13T pinion on 3s = ~50MPH
Mamba X w/1415-2400kv - 16T pinion on 4s = ~57MPH

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.