Well, as the owner of four shapers, I would agree with Bryan above that 1/3 to 1/2 hp should be enough for a shaper the size of the Gingery one. You will be able to cut steel or cast iron OK, bear in mind that most machines are made of cast iron which is not a lot different in strength to aluminium if I remember correctly. The main downside to the aluminium will be that it can be scratched and worn more easily, but for home use with care it will be OK.
You don’t really need a lot of variation in speed for a shaper, none of mine have more than four speeds provided and the smallest one, 6-7 inches stroke has only three speeds. While it is true that you can adjust the speed by changing the stroke, you have to have the stroke long enough to cover the whole length of the job with a bit to spare. Making it longer will increase the cutting speed, but since the tool will be cutting less of the time, there is no net gain. A shaper will usually give a lovely finish, even at low speeds on alloy, so there is no great need to vary the speed a lot. I would agree however that you really want the torque when you reduce the speed, and this tends to mean that you want to change the ratio, not just reduce the speed.
To expand on the above, a motor with a speed controller will be able to produce about the same torque at any speed within its range. So at a quarter of full speed you will get about the same torque as at full speed. If you reduce the speed with pullies or gears, you will get four times the torque when you reduce the speed to a quarter. So for instance although my Myford has a speed controller on it, when I am turning a 6 inch blank for a flywheel, I still engage backgear and drop the speed on the belt as well. That lets me take a heavy cut at a reasonable cutting speed. If I just reduced speed on the controller, I could take a cut at a suitable speed, but not a very heavy one. (Heavy is relative, the Myford does not take what the blokes in industry would call a heavy cut!)
A shaper is not a machine that you would use if you were in a great hurry anyway, which is why they have largely vanished from industry. For the amateur, their charm is the low cost of tooling, and the fact that they are a charming and relaxing machine to watch.
So for the Gingery, I would suggest that you probably do want to stick with a belt drive with stepped pullies or some equivalant. The final reduction, which on commercial shapers is usually a pair of gears, probably want to be a positive drive, eg at the minimum a toothed belt. One of my shapers has a synthetic resin bonded fibre gear for the bull wheel, which suggests to me that a toothed belt drive would stand up to the service OK. A shaper used properly does not impose enormous impact loads on the parts, although it can when accidents happen. In such a case the belt would be good, since it would jump a tooth , or at worst strip the belt, which is cheaper than a gear. I think the SRBF gear in mine may have been intended as a “weakest link”, although an old repair to the slotted arm shows that this did not work out. Now that spare gears are no longer available, it becomes a bit of a worry, although I am sure I could make one if I had to.
Can you mount the motor under the machine, below the top of the bench? It would mean the shaper would become part of the bench, but might help you to keep the bench space small.
regards
John