I know this is an American site, but if it's just piston rings & gaskets, they should be reasonably cheap to post, you might find what you're after here?
I've never used them, so can't reccommend or warn you about them. I just was using the microfiche to try & figure out what I'm about to buy!
If you have no success, I am about to pick up my new RIB, which has the Yam version of this engine on the transom. I have no idea if I'll need anything, but will let you know if I do.....