I heard that dry ice works.
I looked everywhere but couldn't find any large scale smoke units (generators). ???
i really dont know and welcome to the board
I believe Aristo-Craft has some spare smoke units available in their catalogue (http://aristocraft.com/catalog/index.html). At the drop-down box at the lower left of the screen, select "Replacement parts." They are fan-drive.
If you're up for some delicate work, try rigging a crank-operated bellows system driven by the wheels. You'll get realistic puffing action.