In my experience, don't even bother. Places like electricscooterparts.com, surpluscentre.com and others are all over the web, so you should be able to find a type b (with hub and screws) locally. I think modifying the motor is a bad idea, the 63-74 motor that I had had the shaft press fitted and set screwed into the can, so removing it destroyed the press fit, so my new shaft never sat straight.
Also, the shaft is not hardened on the motors (turnigy/leader rc ones) so if that is the motor you have, then the D will rotate around relatively quickly. It may be cheaper in the short run to try to fit the sprocket you have, but I would strongly recommend getting a type b hub. With set screws, you can easily replace the sprockets or take them on/off.
Lastly, be aware that the smaller the sprocket the noisier it will be, especially at high speeds, so if it a 11 tooth, it will be rather noisy at motor speed.