2 ways i know of.
1]. only put 1 motor mount screw in and push the motor over till you get the gears to mesh well.
2]. From what I've been told for a 21 pinion put the motor in the 20 hole and drive the car for a while, after a while the gearboxs frees up and it works fine.

But i would not recommend it for racing. Because of a little thing called rules. In Melbourne (Australia) the rules are for Mini you have run the car how it was designed eg. 20 ,18 ,16 pinion. But if your bashing around in the street no probs.