Ron, any of the Roland or Yamaha sound modules would work for this. They have XM2 & GM2 soundbanks which are what most smf's use. To use the sound engine's premium patches you'll need to edit each SMF to the specific patch you want. If you want a quick method, I know the s-series arrangers have a way of switching GM sounds to the other sounds, AND the BK7m (along with all BK, G and E series keyboards) has the COVER feature which selectively switches out sounds - I have used this for some of my SMFs and it really makes a difference.

If you could find a Roland DisCover5 module, I think you'd be happy with that. Fran had one a while back. A lot of accordion retailers handle Ketron modules; Google 'accordions'; there's a guy in Clearwater, FL who has a great site listing all his stuff for sale online.
http://www.accordionsandkeyboards.net/