There are a couple of stats to look at when considering mechanisms for conversion projects. Steamlocomotive.com is a huge help for this!
The very first thing I look at is driver diameter, and style. Very rarely will you find a mechanism that is spot on, but usually you can find something that has drivers that are very close for your prototype. Does your prototype have spoked or box pox drivers? The J class has spoked, but many of the modern 4-8-4s had box pox.
Secondly on my list is the driver wheelbase. Again, its rare to find something that is dead on, but you can usually find something that is close.
Third on my list is the cylinder diameter and stroke. These numbers will give you the a rough idea of the proportions of the cylinders on your prototype. I usually find myself compromising here.
Fourth and final thing to consider is the valve gear. This is rarely appropriate for my prototype, and I usually will overlook incorrect valve gear if everything else is close.
There are some unique things to the J class that are to be considered, but can be altered. The look of the cylinders, and the lightweight are very distinct. If you're willing to remove the rods, the can easily be reshaped with a jeweler's file. The cylinders might be a bit more involved, but a start would be to remove the "J" class plate on the side.
I hope this helps. Being a WM modeler, I have very few options for steam. But with the advances in 3D printing I have been looking closely at mechanisms to 3D print accurate shells. I have all ready done this with Bachmann's 2-8-0 so I know its possible. I have been considering the Bachmann J for a starting point for a WM Potomac. Keep us posted, and good luck!
