It seemed that the first engine that came up is a personal favorite of mine: the GP30.
My recollection is that the MTH Proto Sound 3.0 GP30 was not offered in the "scale wheels" version with fixed pilot and, well, scale wheels.
I only have experience with one MTH diesel and that is a scale-wheel engine so it needed no conversion.
My understanding is that removing the 3rd rail electrical contact roller is easy. How about replacing those ginormous hi-rail wheels? Does MTH sell a fixed pilot as a part or would it be some surgery and removal of the lobster claw coupler and then attaching the pilot to the shell?