You probably do not need any primer unless the color you want to use is too light and will be changed by the color under it. You can probably use most any paint you desire. MTH uses a very tough paint that likely will not be affected by the new paint. Weaver scale cote paint is good, or Polyscale, Badger, or maybe testors model master paints. There are others as well. If you are painting it black, many different paints will work. If your engine has numbers or lettering you will want to at least sand them smooth so they will not show in the new paint.
Rob