[Max Lobovsky]

Couple ways. You can get a hold of the FP couplers that were in the kit last year. (for some reason, there is an inordinate amount of FP toy cars in the trash. You can find one) it isn't terribly difficult to bolt a sprocket to one of those.

You could also cut slots in a sprocket to mesh to the white piece on the FP gearbox. be careful if you put a lot of tension in the chain though or try to get some more support for the sprocket; I'm not sure about loading that white piece radially.