I just used a pair of needlenose pliers and ever-so-gently twisted the commutator in the proper direction. Since these motors spin clockwise, (as viewed from the pinion gear side) to advance the timing, you would twist the commutator in the clockwise direction. It is important not to use the pliers directly on the commutator contacts, but grip using the collar closer to the armature windings.
You don't need (or want) to twist very far, and I wouldn't do this on a motor that you can't afford to ruin. It is best to experiment on a stage 1 motor or one of those $2.99 jobs that RS sells, until you get the hang of it. Many things can go wrong when you are disassembling and reassembling the motor. You can mess up the brushes, pop the bearings out of the motor can or misalign them, all of which will make you slower than when you started.
Last night, I performed this operation on the X Speed motor, and it now spins faster in the forward (clockwise) direction than reverse.
However, I messed up the brushes a bit, so it isn't running quite as strongly as it should be.
Oh well, time to sacrifice a stage 1 motor... :lol: