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Unfortunately, it's not as simple as just scaling the parts down. If you scale down the parts I've published, you're also scaling down the tolerances / clearances between the parts -- you'd want the tolerances to mostly remain the same while the rest of the model scales. That'll require some CAD work. The models are all available publicly on OnShape (https://cad.onshape.com/documents/57e3f5c529d2af11276b1529/w/eafe4a700fe87f2b71bc3d31/e/f1e5e18535312417d6894ed1).
If you have the ability to make those changes, I would start with the transmission gear sleeves and the zero positioning disc (pay special attention to the teeth -- they must be well defined and sharp enough). The gear sleeves are thin-walled so printing them smaller could be a problem without a smaller nozzle. Be sure to preview what your printer will be doing in your slicer before you go to print anything.
I modified both of those parts to improve the printability with a 0.4mm nozzle at 3:1 scale. Those parts could probably be modified further, but it might require modifying other parts and ultimately lead to redesigning most of the Curta. For instance, the gear sleeves can be widened, but that means that the sleeve may interfere with the step drum's teeth. If that happens, you'll need to move the transmission shafts further away from the step drum a small amount by altering the main casting and the bearing plate. Once that is done, the carriage casting or the digit dials may need modification to be slightly further out from center to match up with the top of the transmission shafts. Then the positioning of each of the carry levers may need to be tweaked and the housing would also need widening to account for the small amount of extra size added to the other parts.
It may be totally possible, but you may also dig yourself a pretty deep hole chasing the rabbit. Now if you have an SLA printer, you don't have to worry about nozzle size and may be able to print those parts at 2:1 scale without going down that rabbit hole. The parts would still probably need to be scaled without scaling the tolerances, though.