I spun the engine coupler, and after looking at pictures I think it is a 18311-A17 ( Has what looks like a splined coupling with a grease zert). Will a 861523A9 replace it? Any help would be appreciated!

Assume you are working with a Bravo? If so check these out and choose. The one with the grease zerk is HD and the other is standard.
http://www.mercstuff.com/couplers.htm

Scroll down to Bravo's.

Thanks for the info, the website really helped.