I think that unless you plan to display it in a museum, scaling it to fit is essential. I'm sure whomever built the boat in the first place would have done the same for you if you were his client.

Someone posted a bit ago on the Qajaq USA forum about scaling a historic design. It was one of the builders who teaches classes. He scales the historic design up or down like you would scale a qajariaq plan to fit the builder. I'm sorry I don't have a link for you. This may be the way to go, but you can make some changes to the frame and leave it the same on the exterior, like if you omit the seeqqortafik and just put in a masik you can probably elongate the cockpit by 4" without changing the overall shape.

It would suck to spend so much time on such a cool boat and not be able to paddle it happily.