I saw there a post along these lines, but was older than 120 days so I will start a new post.   

I think a great 4-8-4 locomotive to offer would the Timken Four Aces which became Northern Pacific 2626.  My reasons are that it would only require a new shell and 73 inch spoked drivers.  I had a friend that pass away 20 years ago, but he wanted to create this engine using a Bachmann chassis.  When we both knew he was not going survive cancer, I asked to buy some of his engines.  Along with these were some parts for future projects he had researched and planned to make; 2626 was one of these engines and a start on a tender. Four Aces/2626 uses the same tender as Bachmann's 4-6-4 J3. The Timken engine demonstrated Timken's roller bearings, as well as, other proprietary steam engine appliances and changed the railroad industry. It demonstrated on the New York Central, and around 15 other major railroads, including C&O, New Haven, Pennsylvania, Northern Pacific, CB&Q, Nickel Plate, MP, as well as others.  It was built as duel service to demonstrate roller bearing benefits for both freight and passenger service.  The Timken engine was finished in a beautiful green with gold pin striping. The NP 2626 was possibly in two color schemes, standard black, and possibly the grey boiler scheme.  With this engine's history and touring much of the America, it would be an engine that would be at home on most model layouts and is a beautiful engine as well. This engine is only currently available in brass and this would be a unique engine and not that far off production wise from Bachmann's current offerings.  One can read more about Four Aces in Frey and Schrenk's book "Northern Pacific Super Steam Era".  Steve

I have old 4-8-4 Daylight, not the pancake motor but not the newest style.  The engine runs great but will not climb a 2% grade with more than 2 cars.  I have several newer style 4-8-4s but would like to modify this to a GS2 engine rather than just discard it.  Has anyone used another non-Bachmann motor in one of these engines?  I am not opposed to doing some modification to the frame. This model has a rather large flywheel with the gearing on the outside of the flywheel.  Seems like this would be gearing the engine pretty tall.  If you have modified one of these I would be interested, or.... are there any tricks to improving the torque of these older motors so modifying the frame would not be necessary?  Steve, near Walla Walla