Sunbrella and most marine canvas will stretch over time. For a really old canvas used as a pattern, the canvas shop will estimate how much smaller to make the new (unstreched ) Bimini. The rule of thumb is 1/8 inch per foot, but on a piece like a Bimini with lots of stiching, darts, curves, and double canvas that has to be adjusted. If it isn't made a little smaller, it will be looser than it should be once it stretches. Location/mating of the zippers is not too hard, and with a fabric like Sunbrella, color match is usually close enough as it doesn't fade too badly. I made a new one a few years ago, using the old one as a pattern. It fit to the side pannels & frames OK, but the boat is only 100 feet from the sewing machine, and I must have made 50 trips back and forth to check as I cut and sewed (sometimes lugging the the pieces of the new top up and down the pier). Like Eric said, it is a hard thing to get right (mine is OK - servicable & doesn't draw negative attention, but it's certainbly not perfect). But, my experience and skill level is no where near that of a good canvas shop. I think they should be able to get you close enough for a servicable Bimini, but be prepared for the initial install to be tough. You will be tugging and stretching (that is normal - visegrips are a help) as the new (unstreched) fabric is mated up with the old (streched) sides. Best to do it on a warm sunny day. You can make small adjustments with the frame and may find it easier to loosen the frame until after you get the top in place.
If the old one is intact, you might be able to get a few more years out of it by restiching, replacing zippers, and waterproofing the fabric.
Regards, Bob K
