I've found two motors I like - both NEMA 23 570 oz-in like the Super Stinger. One is a uni or bipolar motor but only has a 1/4" shaft (420 oz-in in unipolar mode). The other is bipolar only but has a 3/8" shaft. I like the idea of a unipolar as there's more controllers out there for them (and using a smaller drive wheel would compensate for the loss in torque) in case the one I found doesn't work satisfactorly, but 1/4" is getting kinda spindly - I'd prefer a 3/8" shaft. What are your thoughts - do you think the 1/4" would be hefty enough?
Time to make a realistic decision what you want to do with this. If you don't own a fireworks business, how many 12 inch shell do you think you'll make in a year? If the answer is more than 2, you probably should buy a real WASP, since the cost of making experimental 12s to get your own algorithms right will be way more than the cost of the equipment. Be realistic about the size of the motors you need, since this affects everything to do with the construction. The weight of a shell varies with the cube of its diameter, so a 12 inch is 27 times heavier than a 4 inch and 64 times heavier than a 3. So if 570 oz-in motors with 4 inch drive wheels can handle a 12, then you would get the same performance with 11 oz-in motors on 2 inch drive rollers with a 4" (21 oz-in for a 5", 36 oz-in for a 6"). Also bear in mind that the maximum torque is only possible if the drivers can deliver the maximum current, which is likely to be in the region of 15A per phase for big motors. My proto used motors with 1/8 inch shafts delivering 44 oz-in at a modest 1.2 amps per phase.