Stick to the one that is in it - faster motors are available but that won't always give you faster times as the car might be more uncontrollable. Tune the car with its current parts first (you'll be suprised by the diff in perf it makes) and then if you are still not satisfied then consisder the slot-it v12b range of motors in either 26k or 29k (compared to 18k motor in the Renault)
Maybe replace some of the gears and axles, crowns, pinions etc. If this doesn't work, then Yes, you could try a Slot It motor to top it all off. 25K would probably be the best all rounder for corners, straights etc.
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