if your loco is a 0-6-0 , 2-6-0 or 2-6-2 with a smoke unit it should be a simple matter of take old unit out then insert the new smoke unit.
Here is an explosion chart of 0-6-0,2-6-0, 2-6-2 the only difference being the lead and trail trucks.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/51501_HO_Prairie_2-6-2.pdf
replacement smoke units here
http://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=66_68_212&products_id=1706
Thanks to jbsmith