If the smoke unit puffs in sync now you may not need any type of interface with a new motor controller. If you have a Legacy engine now you could swap in this Railsounds board to see if it responds to command signals. Just verify the connectors are the same on both boards. My guess it won't work but you can never be sure with Lionel.
This is what the Legacy Shay Railsounds board looks like for connector comparison. This won't work with ERR though.
Pete