Ron, Ideally someone with both engines could tell you for sure. I was able to find replacements for the Lionel Niagara from a few other sources though so if the Lionel part won't work I am sure you will be able to find a substitute.
The EM1 driveshaft is a telescoping one while the Niagara has a simple non telescoping one. Both can be had from Stock Drive Parts. I ended up using ones I found on ebay for a few bucks. Telescoping ones are there as well. They are used in many RC vehicles.
Pittman motors have 4mm diameter shafts. I suspect the shaft diameter on the worm shaft is also 4 mm. It just a matter of finding one with the right length.
With luck someone can answer your question for sure and provide a simple solution.
Pete
