Hi, I have a 1985 Sierra Supreme just like yours. I've owned it since 2001, and I'm very happy with it. I was curious about it's history and found a link on the web that gave some very interesting facts. Some time ago the two original owners of Mastercraft split ways over a dispute about design. One formed what was then called Ski Supreme, which later became Supreme. Here's a link to the history of Mastercraft that mentions the beginning of Ski Supremes: http://www.answers.com/topic/masterc...-company-inc-1
The current company, Supreme, will not give any history or helpful info because they don't have it available, and they give the impression that they don't have time for any owners like us who want to restore these to original condition. Personally, there customer service is not the best. You'll never get an email response from them, and phone calls are usually fruitless.
I've had the best luck with Discount Inboard Marine (skidim.com) who handles all my mechanical issues. Other items like the windshield, instruments, etc. are from other vendors. Not sure if this helps, but hope it does.