If you had one then there is an excellent Starter motor/alternator repair place in Soton.
As you haven't got one then I think Yamaha starters are made by Hitachi . I was surfing the web one night looking for cheap Yamaha parts and I found a website in America that sold new hitachi branded starter motors for yammie engines.
So do a search for Yammie/hitachi/startermotors/marine on the net and see what you get ( they were suprisingly well priced)
I need a little bit of Rhythym and a little bit of Blues