Re: 1989 Force 50HP Serial Number
I don't think you are gonna be able to find a model number if it isn't on the id tag.
It should look something like this:
HO507F92c
The only model specific parts are going to be the ignition system, the fuel pump, and recirculation systems. These can be id'd by serial number.
Here is a link to a parts page for 1992 40 and 50 hp motors, so you can see how they are id'd.
http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=823616
If the guy at the Mercury parts counter can't find the right parts, do it yourself on the internet.