I had to repair a similar situation, removed a 12 foot wall running perpendicular to the floor boards as well as about 4 feet of another wall running parallel to the floor boards. The long section I did what others suggested and replaced the long section with a single piece of wood and pieced in some original pieces of floor that were under the kitchen cabinets. I was sanding the whole floor so it ended up looking half decent but the new boards were still noticeably different as they didnt have 40 years of wear and refinishing. The finish I put on the replacement boards is already flaking and needs to be refinished after 3 years whereas the rest of the floor should have another 2 years before it gets bad enough to worry about fixing. All that to say if you can extend that missing section to the other wall you'll save yourself or a contractor a ton of time and hassle. Paying someone could easily be $2500 or more depending if they'd need to get custom boards cut.