Castle Creations motor and ESC combo recommendations for the Losi Mini-B:

Micro X2 w/0808-4100kv - Stock Gearing (18/60) on 2s = ~22MPH
Micro X2 w/0808-5300kv - Stock Gearing (18/60) on 2s = ~25MPH
Micro X2 w/1007-6350kv* - Stock Gearing (18/60) on 2s = ~30MPH
Micro X2 w/1007-8450kv* - 16T Pinion / 60 T Spur on 2s = ~40MPH

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

*Note: 1007 installation requires slight modification to edge of chassis. The flare in the chassis will hit the motor, you must use a pair of pliers to flatten out the flare and make the chassis flat at that point. You must also remove and trim back the motor guard.

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.