Castle Creations motor and ESC combo recommendations for the Team Corally Spark XB-6 1/8th scale:

Sidewinder 8th w/1515-2200kv - 13t pinion / 46t spur on 4s = ~45MPH
Monster X w/1515-2200kv - 11t pinion / 46t spur on 6s = ~55MPH
Monster X 8s w/1520-1650kv - 16t pinion / 46t spur on 6s = ~65MPH

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.