Won't fit--D motors are bigger than C motors. IIRC you can go as high as F before you get into restricted territory, but they're not the same size. (And the field has so far managed to successfully self-regulate. Government licenses are not required for the motors, although in the high power realm you may need licenses for some of the pyrotechnics. You do, however, have to content with storage requirements that effectively mean most people can't possess high power motors. You have to buy them at the launch site. Many high power launches also require pre-arranged FAA clearance--they don't want rockets sharing the skies with airplanes.)