Thank you for posting a very clear answer to nitroman's question.
I have checked out the "pictorial diagrams" at the Lionel website, and am persuaded that:
1. the Lionel 600-0103-001 will NOT work in nitroman's application;
2. the Lionel 610-8689-126 MIGHT be made to work, but it doesn't look simple;
3. Both Dallee and QSI make electronic e-units that are direct substitutes for the mechanical e-unit that came with the locomotive, and are sized to fit in the same space and use the existing mounting bracket as the mechanical unit.
I have considerable personal experience with the QSI unit, which I think is designated acru-e, but availability is sometimes problematic. When they are available they have proven excellent with respect to clarity of instructions, ease of installation, and performance. The Dallee model also seems to have a very good reputation -- I just haven't tried one yet.