There are many factors to consider when emulating the rotating horn. The simple things like and 800 hz HPf is basic. The doppler can be done by delaying the horn and modulating the delay. The pan is a basic procedure too. The harder more complicated items are the flare and frequency response of the horn. Unfortunately the most difficult emulation of all is the standing waves produced inside of the cabinet. This seems a near impossible thing to do unless you break the rotation down to fewer degrees of resonant circuits. No you can't take a resonant filter and change it resonant frequency as the horn spins. Because the resonant tones in the cabinet have to die down on their own. So as you can see it takes a lot of simple delays with feedback to pull that off. The bass part of a leslie cabinet with the rotating hole is another chore but like the horn. In fact, many players don't even like the bass section as it makes it difficult to play a smooth base line. So, just food for thought there. The drawbars are the easiest of all,