No need to spend any money. You want the thing to run in the correct direction of rotation (no choice there, the engine dictates that) with the chosen side to earth. Easiest way is to swap the field coil winding ends round so the one that goes to the F terminal on the end-cap goes to the dynamo body and the one to the dynamo body goes to F.
Reason for doing it is up to you - most usual reason would be because you've got an electronic regulator for free and it runs on the opposite polarity? Or you've bought one which has the opposite polarity to what you intended? While the electronic regulator cares about the earth, the dynamo doesn't give a hoot. Easiest test is on the bench, with the dynamo seen to turn as a motor, in the direction you want it to turn as a generator, with the earth of preference. To test you've got it right - neg earth in this case - connect battery neg to dynamo body and positive to a wire that bridges F and D. It should turn the way you want. If you don't bridge F and D, nothing will happen. Nor will it if the dynamo is still connected to its drive system.
There be other ways, but this is my favourite.