Rich, since these are MTH cars, try them first. Do they have aluminum cars that use the same extrusion as your cars? If so, you can probably buy interiors that will work in your cars with internal modifications to adopt them.
My reason for suggesting you contact MTH is that LIONEL's aluminum cars, at least the ones I'm interested in, use the same extrusion which permits use of the interiors introduced in newer production. Most, however, require use of a different frame, and even trucks--so it's not without expense.
