I know late reply but if you are still looking, I was referred by Larson to TaylorMade/Ameritex, who did most original canvas and has specs on hand. I had bimini re-done and very happy.
I bought one for my Larson 186 SEi I/O from http://stores.ebay.com/Doug-Russell-Marine. It is made for that boat and it's a perfect fit. It comes without the snaps installed, so you'll have to do that yourself (or hire an upholstery shop to do it). Great cover, great customer service. They sell a lot of Larson stuff.
Don't know if your sorted yet but if not try: www.sussexboatshop.co.uk/
Look the number up on their site, speak to Mick Mills.
Just made me a cover for my 240 cabrio, great service, very helpful.