I am not sure of the reason that you desire to silence the chuff on the locomotive but it is yours to do with as you wish. You might try enlarging the hole in the smoke unit. Remove the boiler/cab casting, disconnect the piston rod from the gear. At this point remove the smoke unit from the chassis and carefully open the hole at the top of the cylinder to maybe 3/16 inch. That should silence the chuff, but also allow it to puff the smoke.