Hi Mike,
You won't go wrong with Hayden Island Canvas in the Janzen Beach area. We used them years ago with a different boat. Around RCYC (where we keep our boat), their reputation is tops. I look over their work every time I see it on a boat at our marina, or when I've seen their display at the Portland boat show, and no one does better construction or more creative design work than they do.
They charge top dollar though. For what you need, in terms of restitching, you can probably do with a more modestly priced shop. I have experience with only one other canvas shop in the Portland area, but my hunch is several could do your job just fine. We had side curtains made for our aft enclosure once we got the boat to Portland, and because the owners were friends of a friend, we went with Channel Marine ((503) 249-1949). They did a workman like job at a very decent price. Because of some personal circumstances, they got behind on ours and other projects. That wasn't a problem for us. I see their shop doors closed as often as I see them open (I drive by on the way to the boat), but I think they still have a going business.
There is a big difference in the quality of thread that can be used. So ask questions of whatever shop you use about that. The best of all is a Gortex thread (Tenara). It will outlast your canvas, but it is not used as much because it is about 10 times the cost of polyester and polyester dacron (much more common to use for marine canvas). You probably don't want to go with Tenara for resewing just a portion of your canvas. But you might ask about thread size -- 92 is common, but 138 will outlast it by a lot, and ask about it.
- Gini