Jerry,
Are you looking for a particular duplex (T1, Q2, S1) or will any do? Also, you specifically stated MTH or Lionel. 3rd Rail also made nice models of all these you might want to consider, plus they made some interesting variations (skirted or non-skirted). Just a thought.
Bruk's S1 above is a really nice engine if that is what you are looking for.