It's not vintage NOW, but those old Martins weren't in 193x either.
If you do take it on, I'd for sure reinforce the side before cutting any big holes. You can drill a small one, and use that to pull a patch up tight on the inside. For that you could either shape a block exactly, which would be tricky, or shape one 'pretty close' to use, with some padding, as a caul for something else. I've used 1/32" plywood from the model airplane shop as a reinforcement several times. It's flexible enough to follow the curve, and quite strong. Epoxy it in with the face veneer across the grain of the side. Be sure to use a pieceof plastic bag on the caul: I know I don't need to tell you, but we all forget sometimes.....
Staying with a small port, and keeping it fairly close to the main soundhole location, will avoid large changes in the timbre. The 'main air' mode pitch will rise, and if it's already high that could lead to problems in the sound of the lowest notes. I'd chenck that before hand, and if it's already higher than, say, G#, I'd want to have some plan in place to drop it. You could do that by adding some sort of sleeve in the soundhole. That's another good use for the thin plywood. It might even work to add a tube to the port: I'll have to check that out when I get some time.
The hole can either be drilled with a Forstner bit and a backup block on the inside, or cut out with a Dremel/handshaft. One of my students put a port on a reso he made; the sound was originally 'choked' by a small soundhole. He used his handshaft to cut the hole in the side, and kept enlarging it until it started to sound worse. Then he put in a trim ring, which covered up the small irregularities around the rim and made the hole just enough smaller to get back into 'good' territory again. This would be an especially good way to go if you could do it when the owner was there; let HIM figure out how big he wants the hole. Then he's got less room to complain later.
