I know this is a commonly brought up topic, but the N scale 4-4-0s can be remade with a motor in the boiler/cab, I am currently reconstructing my first 4-4-0 to have a coreless motor in it's boiler. The biggest problem I'm facing is the wheel alignment, they keep jamming & I think they aren't quartered correctly. I'll try to fix the quartering issue, but if it doesn't work I'll update this post & ask for suggestions. On the other hand I have to rebuild the tender from the ground up because the old one is broken.
This is a very interesting topic. I would imagine by pulling the motor out of the original location and insert it into the tender, it would cause the front of the locomotive to be too light? Some king of weight might be needed.
Can you post a photo of the 4-4-0 and please keep us posted!