I purchased a couple of TMCC conversion boards for MTH PS1 in the early 2000's. As I recall, there were two stacked boards in the locomotives-one was for sound, the other was essentially the E-Unit (I know I am over simplifying this and probably not getting it quite right, but it has been a very long time).
You removed the sound board and it plugged directly into the TMCC conversion board (stacked like the original).
These worked pretty well, but the company who made them went out of business a very long time ago.