I had my valve cover polished and the company broke the vent pipe that comes out the top, see pic below. I noticed there are 18 mm threads in the opening and figured someone must have figured a way to fix it. Any ideas?

Just order a new one :

Part #: 13267-23000

You'd have to polish it, of course, but the shop that broke your original should be happy to do that at no charge.

Too easy, didn't know the part still existed! Thanks, Zup!!