I might be wrong but AFAIK the hornsresonant frequency is dictated mostly be the inertia of the moving part. As such you wont get the same pitch using a smaller generator.
I suggest the most feasible way to get as close to the sound youre looking for while leaving the volume just a matter of how much you're willing to pay, is a recording and (a) large speaker(s) with a mock horn to look the part.