Though I've never built a model siren, I can help with the math. Divide 2840rpm by the number of ports to get the frequency, then divide 3600 (or whatever the exact rpm is) by the frequency and round to the nearest whole number to get the most accurate-sounding number of ports.
The tough part is that, with dual tones, the ratio between their port numbers has to be the same. So with a 10/12 siren, the ratio is 5:6 (a minor third), so you'd have to make sure that the new port ratio also divides evenly into 5:6.
If there's no good way to make the math work out on that, it may be best to simply use the original port numbers and accept a higher tone than the real thing has. Or, err on the side of a lower tone than original, which would certainly be (slightly) easier on the ears.
Historic preservation college student in northeastern West Virginia.
Interested in the acoustical properties of mechanical sirens from a scientific and musical perspective.
West Virginia State-wide & Western Maryland Siren Map