Scott, the problem is no one now has a solution to use a motor larger than a 385. To use anything larger, like a 555, requires modifying the existing frame, and maybe the body of the locomotive as well.
I was able to do one. This will give you an idea what was involved. Maybe someone else could come up with a simpler way. If they do and a kit could be put together both the motors and the ERR kits would be flying out the door.
Show this to your engine builders. They may be able to help.
Pete
