I must be missing something obvious here, but... isn't that the default behavior ? What results do you get when you run this:
For me, I then get a non-stretched full-screen (not windowed) 'original game resolution', including artwork if you have them. (If you have the art but still don't want to show it, I guess you could point your artpath to something that doesn't have any art, for example adding:)
Does this get you what you are looking for ? If so, you need to look at some options in your mame.ini and change them. If not, I have no idea what else to try.