Mate you need to be clear when asking for advice otherwise we cant help.
Are you trying to achieve variable speed during roasting....as in.....to be able to vary the speed of the motor/drum at will during the roast, OR are you just trying to change the speed of the drum period. If so, that can be done with gearing / pulleys / belts.
If you are trying to vary the speed of the drum at will during roasting, do you have the right type of electric motor that will withstand that kind of control? It is not as simple as varying input voltage..... And of course this all falls under the classification of "its only money"......as long as the cost of the mod can be justified.
Additionally, if you vary the speed of the drum during roasting, what are you going to do with the heating and air speed (if you have a machine that can vary air speed), because to just vary the drum speed without being able to account for the others may well be detrimental to the roast process...
This all falls under the objective of ***what are you trying to achieve?***.
Suggest you talk to an electrical engineer and / or electrical fitter, with a thorough explanation of what your current roasting plant set up is, because these questions cannot be answered simply without knowledge what your set up is like. You may also need to speak to a roasting plant engineer.
Hope that helps.