Castle Creations motor and ESC combo recommendations for the Losi JRX2:

Micro X2 w/0808-5300kv - Stock Gearing (17/60) on 2s = ~16MPH
Micro X2 w/0808-4100kv - Stock Gearing (17/60) on 3s = ~19MPH
Micro X2 w/0808-8200kv - Stock Gearing (17/60) on 2s = ~25MPH

Note: Requires aftermarket receiver and transmitter to replace stock 2-n-1 ESC/receiver combo.

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.