Castle Creations motor and ESC combo recommendations for the Rlaarlo AK-987:

Copperhead 10 w/1410-3800kv - 30T pinion / 78T spur on 3s = ~40MPH
Mamba X w/1412-3200kv - 36T pinion / 63T spur on 3s = ~50MPH
Cobra 10 w/1415-2400kv - 42T pinion / 63T spur on 4s = ~60MPH

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.