I remembered getting a home movie from a guy who went to the U.S. in 1989 & went to a few nationals & supercross's, I just got it out & had a look,he done a walk around of Damon Bradshaws 89 YZ125W,although the rules were production based the factory teams still used some works parts, I noticed it had billet triples & hubs,special forks & a cone type pipe,the pipe didn't appear to have any brand markings on it,could have been a factory part, the muffler looked stock,maybe shortened though,undoubtedly there would have been more trick parts on & in the bike that you couldn't see.
I don't know anything about what they ran in Europe that year regarding the little y zee,As far as getting a works pipe i dont know, Have you tried putting a wanted add on Marks swapmeet?